import java.io.*;

public class Main {

	final static long MOD = 1000000007;
	public static String[] Ws = null;
	public static int wsIndx = 0;
	public static BufferedReader in = null;
	public static void main(String[] args) throws Exception {


		String s = nextString();
		int[] o = {0,0};
		int[] e = {0,0};

		for(int i=0;i<s.length();i++){
			if(s.charAt(i)=='0'){
				if(i%2==0){
					e[0]++;
				}else{
					o[0]++;
				}
			}else{
				if(i%2==0){
					e[1]++;
				}else{
					o[1]++;
				}
			}
		}
		int ans =0;
		if(o[0]>=o[1]){
			if(o[0]>=e[0]){
				ans=e[0]+o[1];
			}else{
				ans=o[0]+e[1];
			}
		}else{
			if(o[1]>=e[1]){
				ans=e[1]+o[0];
			}else{
				ans=o[1]+e[0];
			}
		}
		System.out.println(ans);

	}


	public static void check() throws Exception{
		if(in == null){
			in = new BufferedReader(new InputStreamReader(System.in));
		}
		if(Ws==null || Ws.length<=wsIndx){
			Ws = in.readLine().split(" ");
			wsIndx=0;
		}
	}
	public static int nextInt()throws Exception{
		check();
		return Integer.parseInt(Ws[wsIndx++]);
	}

	public static long nextLong()throws Exception{
		check();
		return Long.parseLong(Ws[wsIndx++]);
	}

	public static String nextString()throws Exception{
		check();
		return Ws[wsIndx++];
	}

	public static int[] nextInts()throws Exception{
		check();
		int[] tmp = new int[Ws.length];
		for(int i=0;i<tmp.length;i++){
			tmp[i]=Integer.parseInt(Ws[i]);
		}
		wsIndx=Ws.length;
		return tmp;
	}

	public static long[] nextLongs()throws Exception{
		check();
		long[] tmp = new long[Ws.length];
		for(int i=0;i<tmp.length;i++){
			tmp[i]=Long.parseLong(Ws[i]);
		}
		wsIndx=Ws.length;
		return tmp;
	}

	public static String[] nextStrings()throws Exception{
		check();
		wsIndx=Ws.length;
		return Ws;
	}


} import java.io.*; io.* java public class Main {

	final static long MOD = 1000000007;
	public static String[] Ws = null;
	public static int wsIndx = 0;
	public static BufferedReader in = null;
	public static void main(String[] args) throws Exception {


		String s = nextString();
		int[] o = {0,0};
		int[] e = {0,0};

		for(int i=0;i<s.length();i++){
			if(s.charAt(i)=='0'){
				if(i%2==0){
					e[0]++;
				}else{
					o[0]++;
				}
			}else{
				if(i%2==0){
					e[1]++;
				}else{
					o[1]++;
				}
			}
		}
		int ans =0;
		if(o[0]>=o[1]){
			if(o[0]>=e[0]){
				ans=e[0]+o[1];
			}else{
				ans=o[0]+e[1];
			}
		}else{
			if(o[1]>=e[1]){
				ans=e[1]+o[0];
			}else{
				ans=o[1]+e[0];
			}
		}
		System.out.println(ans);

	}


	public static void check() throws Exception{
		if(in == null){
			in = new BufferedReader(new InputStreamReader(System.in));
		}
		if(Ws==null || Ws.length<=wsIndx){
			Ws = in.readLine().split(" ");
			wsIndx=0;
		}
	}
	public static int nextInt()throws Exception{
		check();
		return Integer.parseInt(Ws[wsIndx++]);
	}

	public static long nextLong()throws Exception{
		check();
		return Long.parseLong(Ws[wsIndx++]);
	}

	public static String nextString()throws Exception{
		check();
		return Ws[wsIndx++];
	}

	public static int[] nextInts()throws Exception{
		check();
		int[] tmp = new int[Ws.length];
		for(int i=0;i<tmp.length;i++){
			tmp[i]=Integer.parseInt(Ws[i]);
		}
		wsIndx=Ws.length;
		return tmp;
	}

	public static long[] nextLongs()throws Exception{
		check();
		long[] tmp = new long[Ws.length];
		for(int i=0;i<tmp.length;i++){
			tmp[i]=Long.parseLong(Ws[i]);
		}
		wsIndx=Ws.length;
		return tmp;
	}

	public static String[] nextStrings()throws Exception{
		check();
		wsIndx=Ws.length;
		return Ws;
	}


} Main final static long MOD = 1000000007; MOD 1000000007 public static String[] Ws = null; Ws null public static int wsIndx = 0; wsIndx 0 public static BufferedReader in = null; in null public static void main(String[] args) throws Exception {


		String s = nextString();
		int[] o = {0,0};
		int[] e = {0,0};

		for(int i=0;i<s.length();i++){
			if(s.charAt(i)=='0'){
				if(i%2==0){
					e[0]++;
				}else{
					o[0]++;
				}
			}else{
				if(i%2==0){
					e[1]++;
				}else{
					o[1]++;
				}
			}
		}
		int ans =0;
		if(o[0]>=o[1]){
			if(o[0]>=e[0]){
				ans=e[0]+o[1];
			}else{
				ans=o[0]+e[1];
			}
		}else{
			if(o[1]>=e[1]){
				ans=e[1]+o[0];
			}else{
				ans=o[1]+e[0];
			}
		}
		System.out.println(ans);

	} main {


		String s = nextString();
		int[] o = {0,0};
		int[] e = {0,0};

		for(int i=0;i<s.length();i++){
			if(s.charAt(i)=='0'){
				if(i%2==0){
					e[0]++;
				}else{
					o[0]++;
				}
			}else{
				if(i%2==0){
					e[1]++;
				}else{
					o[1]++;
				}
			}
		}
		int ans =0;
		if(o[0]>=o[1]){
			if(o[0]>=e[0]){
				ans=e[0]+o[1];
			}else{
				ans=o[0]+e[1];
			}
		}else{
			if(o[1]>=e[1]){
				ans=e[1]+o[0];
			}else{
				ans=o[1]+e[0];
			}
		}
		System.out.println(ans);

	} String s = nextString(); s nextString() nextString int[] o = {0,0}; o {0,0} 0 0 int[] e = {0,0}; e {0,0} 0 0 for(int i=0;i<s.length();i++){
			if(s.charAt(i)=='0'){
				if(i%2==0){
					e[0]++;
				}else{
					o[0]++;
				}
			}else{
				if(i%2==0){
					e[1]++;
				}else{
					o[1]++;
				}
			}
		} int i=0; int i=0; i 0 i<s.length() i s.length() s.length s i++ i++ i {
			if(s.charAt(i)=='0'){
				if(i%2==0){
					e[0]++;
				}else{
					o[0]++;
				}
			}else{
				if(i%2==0){
					e[1]++;
				}else{
					o[1]++;
				}
			}
		} {
			if(s.charAt(i)=='0'){
				if(i%2==0){
					e[0]++;
				}else{
					o[0]++;
				}
			}else{
				if(i%2==0){
					e[1]++;
				}else{
					o[1]++;
				}
			}
		} if(s.charAt(i)=='0'){
				if(i%2==0){
					e[0]++;
				}else{
					o[0]++;
				}
			}else{
				if(i%2==0){
					e[1]++;
				}else{
					o[1]++;
				}
			} s.charAt(i)=='0' s.charAt(i) s.charAt s i '0' {
				if(i%2==0){
					e[0]++;
				}else{
					o[0]++;
				}
			} if(i%2==0){
					e[0]++;
				}else{
					o[0]++;
				} i%2==0 i%2 i 2 0 {
					e[0]++;
				} e[0]++ e[0] e 0 {
					o[0]++;
				} o[0]++ o[0] o 0 {
				if(i%2==0){
					e[1]++;
				}else{
					o[1]++;
				}
			} if(i%2==0){
					e[1]++;
				}else{
					o[1]++;
				} i%2==0 i%2 i 2 0 {
					e[1]++;
				} e[1]++ e[1] e 1 {
					o[1]++;
				} o[1]++ o[1] o 1 int ans =0; ans 0 if(o[0]>=o[1]){
			if(o[0]>=e[0]){
				ans=e[0]+o[1];
			}else{
				ans=o[0]+e[1];
			}
		}else{
			if(o[1]>=e[1]){
				ans=e[1]+o[0];
			}else{
				ans=o[1]+e[0];
			}
		} o[0]>=o[1] o[0] o 0 o[1] o 1 {
			if(o[0]>=e[0]){
				ans=e[0]+o[1];
			}else{
				ans=o[0]+e[1];
			}
		} if(o[0]>=e[0]){
				ans=e[0]+o[1];
			}else{
				ans=o[0]+e[1];
			} o[0]>=e[0] o[0] o 0 e[0] e 0 {
				ans=e[0]+o[1];
			} ans=e[0]+o[1] ans e[0]+o[1] e[0] e 0 o[1] o 1 {
				ans=o[0]+e[1];
			} ans=o[0]+e[1] ans o[0]+e[1] o[0] o 0 e[1] e 1 {
			if(o[1]>=e[1]){
				ans=e[1]+o[0];
			}else{
				ans=o[1]+e[0];
			}
		} if(o[1]>=e[1]){
				ans=e[1]+o[0];
			}else{
				ans=o[1]+e[0];
			} o[1]>=e[1] o[1] o 1 e[1] e 1 {
				ans=e[1]+o[0];
			} ans=e[1]+o[0] ans e[1]+o[0] e[1] e 1 o[0] o 0 {
				ans=o[1]+e[0];
			} ans=o[1]+e[0] ans o[1]+e[0] o[1] o 1 e[0] e 0 System.out.println(ans) System.out.println System.out System System.out ans String[] args args public static void check() throws Exception{
		if(in == null){
			in = new BufferedReader(new InputStreamReader(System.in));
		}
		if(Ws==null || Ws.length<=wsIndx){
			Ws = in.readLine().split(" ");
			wsIndx=0;
		}
	} check {
		if(in == null){
			in = new BufferedReader(new InputStreamReader(System.in));
		}
		if(Ws==null || Ws.length<=wsIndx){
			Ws = in.readLine().split(" ");
			wsIndx=0;
		}
	} if(in == null){
			in = new BufferedReader(new InputStreamReader(System.in));
		} in == null in null {
			in = new BufferedReader(new InputStreamReader(System.in));
		} in = new BufferedReader(new InputStreamReader(System.in)) in new BufferedReader(new InputStreamReader(System.in)) if(Ws==null || Ws.length<=wsIndx){
			Ws = in.readLine().split(" ");
			wsIndx=0;
		} Ws==null || Ws.length<=wsIndx Ws==null Ws null Ws.length<=wsIndx Ws.length Ws Ws.length wsIndx {
			Ws = in.readLine().split(" ");
			wsIndx=0;
		} Ws = in.readLine().split(" ") Ws in.readLine().split(" ") in.readLine().split in.readLine() in.readLine in " " wsIndx=0 wsIndx 0 public static int nextInt()throws Exception{
		check();
		return Integer.parseInt(Ws[wsIndx++]);
	} nextInt {
		check();
		return Integer.parseInt(Ws[wsIndx++]);
	} check() check return Integer.parseInt(Ws[wsIndx++]); Integer.parseInt(Ws[wsIndx++]) Integer.parseInt Integer Ws[wsIndx++] Ws wsIndx++ wsIndx public static long nextLong()throws Exception{
		check();
		return Long.parseLong(Ws[wsIndx++]);
	} nextLong {
		check();
		return Long.parseLong(Ws[wsIndx++]);
	} check() check return Long.parseLong(Ws[wsIndx++]); Long.parseLong(Ws[wsIndx++]) Long.parseLong Long Ws[wsIndx++] Ws wsIndx++ wsIndx public static String nextString()throws Exception{
		check();
		return Ws[wsIndx++];
	} nextString {
		check();
		return Ws[wsIndx++];
	} check() check return Ws[wsIndx++]; Ws[wsIndx++] Ws wsIndx++ wsIndx public static int[] nextInts()throws Exception{
		check();
		int[] tmp = new int[Ws.length];
		for(int i=0;i<tmp.length;i++){
			tmp[i]=Integer.parseInt(Ws[i]);
		}
		wsIndx=Ws.length;
		return tmp;
	} nextInts {
		check();
		int[] tmp = new int[Ws.length];
		for(int i=0;i<tmp.length;i++){
			tmp[i]=Integer.parseInt(Ws[i]);
		}
		wsIndx=Ws.length;
		return tmp;
	} check() check int[] tmp = new int[Ws.length]; tmp new int[Ws.length] Ws.length Ws Ws.length for(int i=0;i<tmp.length;i++){
			tmp[i]=Integer.parseInt(Ws[i]);
		} int i=0; int i=0; i 0 i<tmp.length i tmp.length tmp tmp.length i++ i++ i {
			tmp[i]=Integer.parseInt(Ws[i]);
		} {
			tmp[i]=Integer.parseInt(Ws[i]);
		} tmp[i]=Integer.parseInt(Ws[i]) tmp[i] tmp i Integer.parseInt(Ws[i]) Integer.parseInt Integer Ws[i] Ws i wsIndx=Ws.length wsIndx Ws.length Ws Ws.length return tmp; tmp public static long[] nextLongs()throws Exception{
		check();
		long[] tmp = new long[Ws.length];
		for(int i=0;i<tmp.length;i++){
			tmp[i]=Long.parseLong(Ws[i]);
		}
		wsIndx=Ws.length;
		return tmp;
	} nextLongs {
		check();
		long[] tmp = new long[Ws.length];
		for(int i=0;i<tmp.length;i++){
			tmp[i]=Long.parseLong(Ws[i]);
		}
		wsIndx=Ws.length;
		return tmp;
	} check() check long[] tmp = new long[Ws.length]; tmp new long[Ws.length] Ws.length Ws Ws.length for(int i=0;i<tmp.length;i++){
			tmp[i]=Long.parseLong(Ws[i]);
		} int i=0; int i=0; i 0 i<tmp.length i tmp.length tmp tmp.length i++ i++ i {
			tmp[i]=Long.parseLong(Ws[i]);
		} {
			tmp[i]=Long.parseLong(Ws[i]);
		} tmp[i]=Long.parseLong(Ws[i]) tmp[i] tmp i Long.parseLong(Ws[i]) Long.parseLong Long Ws[i] Ws i wsIndx=Ws.length wsIndx Ws.length Ws Ws.length return tmp; tmp public static String[] nextStrings()throws Exception{
		check();
		wsIndx=Ws.length;
		return Ws;
	} nextStrings {
		check();
		wsIndx=Ws.length;
		return Ws;
	} check() check wsIndx=Ws.length wsIndx Ws.length Ws Ws.length return Ws; Ws public class Main {

	final static long MOD = 1000000007;
	public static String[] Ws = null;
	public static int wsIndx = 0;
	public static BufferedReader in = null;
	public static void main(String[] args) throws Exception {


		String s = nextString();
		int[] o = {0,0};
		int[] e = {0,0};

		for(int i=0;i<s.length();i++){
			if(s.charAt(i)=='0'){
				if(i%2==0){
					e[0]++;
				}else{
					o[0]++;
				}
			}else{
				if(i%2==0){
					e[1]++;
				}else{
					o[1]++;
				}
			}
		}
		int ans =0;
		if(o[0]>=o[1]){
			if(o[0]>=e[0]){
				ans=e[0]+o[1];
			}else{
				ans=o[0]+e[1];
			}
		}else{
			if(o[1]>=e[1]){
				ans=e[1]+o[0];
			}else{
				ans=o[1]+e[0];
			}
		}
		System.out.println(ans);

	}


	public static void check() throws Exception{
		if(in == null){
			in = new BufferedReader(new InputStreamReader(System.in));
		}
		if(Ws==null || Ws.length<=wsIndx){
			Ws = in.readLine().split(" ");
			wsIndx=0;
		}
	}
	public static int nextInt()throws Exception{
		check();
		return Integer.parseInt(Ws[wsIndx++]);
	}

	public static long nextLong()throws Exception{
		check();
		return Long.parseLong(Ws[wsIndx++]);
	}

	public static String nextString()throws Exception{
		check();
		return Ws[wsIndx++];
	}

	public static int[] nextInts()throws Exception{
		check();
		int[] tmp = new int[Ws.length];
		for(int i=0;i<tmp.length;i++){
			tmp[i]=Integer.parseInt(Ws[i]);
		}
		wsIndx=Ws.length;
		return tmp;
	}

	public static long[] nextLongs()throws Exception{
		check();
		long[] tmp = new long[Ws.length];
		for(int i=0;i<tmp.length;i++){
			tmp[i]=Long.parseLong(Ws[i]);
		}
		wsIndx=Ws.length;
		return tmp;
	}

	public static String[] nextStrings()throws Exception{
		check();
		wsIndx=Ws.length;
		return Ws;
	}


} public class Main {

	final static long MOD = 1000000007;
	public static String[] Ws = null;
	public static int wsIndx = 0;
	public static BufferedReader in = null;
	public static void main(String[] args) throws Exception {


		String s = nextString();
		int[] o = {0,0};
		int[] e = {0,0};

		for(int i=0;i<s.length();i++){
			if(s.charAt(i)=='0'){
				if(i%2==0){
					e[0]++;
				}else{
					o[0]++;
				}
			}else{
				if(i%2==0){
					e[1]++;
				}else{
					o[1]++;
				}
			}
		}
		int ans =0;
		if(o[0]>=o[1]){
			if(o[0]>=e[0]){
				ans=e[0]+o[1];
			}else{
				ans=o[0]+e[1];
			}
		}else{
			if(o[1]>=e[1]){
				ans=e[1]+o[0];
			}else{
				ans=o[1]+e[0];
			}
		}
		System.out.println(ans);

	}


	public static void check() throws Exception{
		if(in == null){
			in = new BufferedReader(new InputStreamReader(System.in));
		}
		if(Ws==null || Ws.length<=wsIndx){
			Ws = in.readLine().split(" ");
			wsIndx=0;
		}
	}
	public static int nextInt()throws Exception{
		check();
		return Integer.parseInt(Ws[wsIndx++]);
	}

	public static long nextLong()throws Exception{
		check();
		return Long.parseLong(Ws[wsIndx++]);
	}

	public static String nextString()throws Exception{
		check();
		return Ws[wsIndx++];
	}

	public static int[] nextInts()throws Exception{
		check();
		int[] tmp = new int[Ws.length];
		for(int i=0;i<tmp.length;i++){
			tmp[i]=Integer.parseInt(Ws[i]);
		}
		wsIndx=Ws.length;
		return tmp;
	}

	public static long[] nextLongs()throws Exception{
		check();
		long[] tmp = new long[Ws.length];
		for(int i=0;i<tmp.length;i++){
			tmp[i]=Long.parseLong(Ws[i]);
		}
		wsIndx=Ws.length;
		return tmp;
	}

	public static String[] nextStrings()throws Exception{
		check();
		wsIndx=Ws.length;
		return Ws;
	}


} Main
import java.io.*;

public class Main {

	final static long MOD = 1000000007;
	public static String[] Ws = null;
	public static int wsIndx = 0;
	public static BufferedReader in = null;
	public static void main(String[] args) throws Exception {


		String s = nextString();
		int[] o = {0,0};
		int[] e = {0,0};

		for(int i=0;i<s.length();i++){
			if(s.charAt(i)=='0'){
				if(i%2==0){
					e[0]++;
				}else{
					o[0]++;
				}
			}else{
				if(i%2==0){
					e[1]++;
				}else{
					o[1]++;
				}
			}
		}
		int ans =0;
		if(o[0]>=o[1]){
			if(o[0]>=e[0]){
				ans=e[0]+o[1];
			}else{
				ans=o[0]+e[1];
			}
		}else{
			if(o[1]>=e[1]){
				ans=e[1]+o[0];
			}else{
				ans=o[1]+e[0];
			}
		}
		System.out.println(ans);

	}


	public static void check() throws Exception{
		if(in == null){
			in = new BufferedReader(new InputStreamReader(System.in));
		}
		if(Ws==null || Ws.length<=wsIndx){
			Ws = in.readLine().split(" ");
			wsIndx=0;
		}
	}
	public static int nextInt()throws Exception{
		check();
		return Integer.parseInt(Ws[wsIndx++]);
	}

	public static long nextLong()throws Exception{
		check();
		return Long.parseLong(Ws[wsIndx++]);
	}

	public static String nextString()throws Exception{
		check();
		return Ws[wsIndx++];
	}

	public static int[] nextInts()throws Exception{
		check();
		int[] tmp = new int[Ws.length];
		for(int i=0;i<tmp.length;i++){
			tmp[i]=Integer.parseInt(Ws[i]);
		}
		wsIndx=Ws.length;
		return tmp;
	}

	public static long[] nextLongs()throws Exception{
		check();
		long[] tmp = new long[Ws.length];
		for(int i=0;i<tmp.length;i++){
			tmp[i]=Long.parseLong(Ws[i]);
		}
		wsIndx=Ws.length;
		return tmp;
	}

	public static String[] nextStrings()throws Exception{
		check();
		wsIndx=Ws.length;
		return Ws;
	}


}
