166
rbbbbbbbbbbbbrbrrbbrbbbrbbbbbbbbbbrbbbbbbrbbbrbbbbbrbbbbbbbrbbbbbbbrbbrbbbbbbbbrbbbbbbbbbbbbbbrrbbbrbbbbbbbbbbbbbbrbrbbbbbbbbbbbrbbbbbbbbbbbbbbrbbbbbbbbbbbbbbbbbbbbbb
 7
rrbrbrb
 1
r
 2
bb
 18
rrrrrrrrrrrrrrrrrb
 8
rbbrbrbr
 1
b
 13
rbbbrbrrbrrbb
 100
brbbbrrrbbrbrbbrbbrbbbbrbbrrbbbrrbbbbrbrbbbbbbbbbbbbbbbbrrrrbbbbrrrbbbbbbbrbrrbrbbbbrrrbbbbrbbrbbbrb
 2
rb
 2
rr
 2
br

70
 1
 0
 1
 8
 1
 0
 3
 34
 0
 1
 0

import java.io.*;
import java.util.Arrays;
import java.util.Random;

public class Div_372_1{
public static void main(String[] args) throws IOException {
	IOFast s=new IOFast();
	PrintWriter out=new PrintWriter(new OutputStreamWriter(System.out));
	int t=1;
	
	while(t-->0)
	{
		int n=s.nextInt();
		char c[]=s.next();
		char a[]=new char[n];
		char b[]=new char[n];
		int an1=0;
		int an2=0;
		int countr=0;
		int countb=0;
		
		for(int i=0;i<n;i++)
		{
		
			if(c[i]=='r')countr++;
			if(c[i]=='b')countb++;
			
			if(i%2==0)
			{
					a[i]='r';
					b[i]='b';
			}
				else
			{
				a[i]='b';
				b[i]='r';
			}
		}
		int req=(n+1)/2 ;
		int req2=n/2;
		int req3=n/2+1;
		int req4=n/2;
		int a1=Math.abs(countr-req);
		int a2=Math.abs(countb-req);
		int k1=0;
		int k2=0;
		for(int i=0;i<n;i++)
		{
			a[i]=c[i];
			if(c[i]=='b' && i%2==0 && k1<a1)
			{
				a[i]='r';
				k1++;
				//if(k1==a1)break;
			}
			if(c[i]=='r' && i%2==1 && k1<a1)
			{
				a[i]='b';
				k1++;
				//if(k1==a1)break;
			}
		}
		for(int i=0;i<n;i++)
		{
			b[i]=c[i];
			if(c[i]=='r' && i%2==0 && k2<a2)
			{
				b[i]='b';
				k2++;
			//	if(k2==a2)break;
			}
			if(c[i]=='b' && i%2==1 && k2<a2)
			{
				b[i]='r';
				k2++;
			//	if(k2==a2)break;
			}
	
		}
		int fans=0;
		String st=String.valueOf(b);
		int ans1=0;
		int ans2=0;
		for(int i=0;i<n;i++)
		{
		if(st.charAt(i)=='r' && i%2==0)
		{
			ans1++;
		}
		if(st.charAt(i)=='b' && i%2==1)
		{
			ans1++;
		}
		if(st.charAt(i)=='r' && i%2==1)
		{
			ans2++;
		}
		if(st.charAt(i)=='b' && i%2==0)
		{
			ans2++;
		}
		

		}
		ans1=(ans1+1)/2;
		ans2=(ans2+1)/2;
		int ans=Math.min(ans1,ans2);
		ans+=a2;
	    st=String.valueOf(a);
	    ans1=0;
	    ans2=0;
	for(int i=0;i<n;i++)
	{
	if(st.charAt(i)=='r' && i%2==0)
	{
		ans1++;
	}
	if(st.charAt(i)=='b' && i%2==1)
	{
		ans1++;
	}
	if(st.charAt(i)=='r' && i%2==1)
	{
		ans2++;
	}
	if(st.charAt(i)=='b' && i%2==0)
	{
		ans2++;
	}
	

	}
	
	ans1=(ans1+1)/2;
	ans2=(ans2+1)/2;
	
	ans=Math.min(ans,Math.min(ans1,ans2)+a1);
  	out.println(ans);
    }




	
		
	
	
	

	
	out.close();

}
static int MOD=(int)(1e9+7);

static int dp[];
public static void shuffle(int[] a) {
	Random rnd = new Random();
	for (int i = a.length - 1; i >= 1; i--) {
		int j = rnd.nextInt(i + 1);
		int t = a[i];
		a[i] = a[j];
		a[j] = t;
	}
}

static double dist(int x[],int y[],int u,int v)
{
	return Math.sqrt(Math.pow((x[u]-x[v]),2)+Math.pow((y[u]-y[v]),2));
}
static int gcd(int n, int r) { return r == 0 ? n : gcd(r, n%r); }
	static long gcd(long n, long r) { return r == 0 ? n : gcd(r, n%r); }
	
	static <T> void swap(T[] x, int i, int j) { T t = x[i]; x[i] = x[j]; x[j] = t; }
	static void swap(int[] x, int i, int j) { int t = x[i]; x[i] = x[j]; x[j] = t; }
	
	
}	
class EndOfFileRuntimeException extends RuntimeException {
	private static final long serialVersionUID = -8565341110209207657L; }

class IOFast {
		private BufferedReader in = new BufferedReader(new InputStreamReader(System.in));
		private PrintWriter out = new PrintWriter(System.out);

		void setFileIn(String ins) throws IOException { in.close(); in = new BufferedReader(new FileReader(ins)); }
		void setFileOut(String outs) throws IOException { out.flush(); out.close(); out = new PrintWriter(new FileWriter(outs)); }
		void setFileIO(String ins, String outs) throws IOException { setFileIn(ins); setFileOut(outs); }

		private static int pos, readLen;
		private static final char[] buffer = new char[1024 * 8];
		private static char[] str = new char[500*8*2];
		private static boolean[] isDigit = new boolean[256];
		private static boolean[] isSpace = new boolean[256];
		private static boolean[] isLineSep = new boolean[256];

		static { for(int i = 0; i < 10; i++) { isDigit['0' + i] = true; } isDigit['-'] = true; isSpace[' '] = isSpace['\r'] = isSpace['\n'] = isSpace['\t'] = true; isLineSep['\r'] = isLineSep['\n'] = true; }
		public int read() throws IOException { if(pos >= readLen) { pos = 0; readLen = in.read(buffer); if(readLen <= 0) { throw new EndOfFileRuntimeException(); } } return buffer[pos++]; }

		public int nextInt() throws IOException { int len = 0; str[len++] = nextChar(); len = reads(len, isSpace); int i = 0; int ret = 0; if(str[0] == '-') { i = 1; } for(; i < len; i++) ret = ret * 10 + str[i] - '0'; if(str[0] == '-') { ret = -ret; } return ret; }
		public long nextLong() throws IOException { int len = 0; str[len++] = nextChar(); len = reads(len, isSpace); int i = 0; long ret = 0; if(str[0] == '-') { i = 1; } for(; i < len; i++) ret = ret * 10 + str[i] - '0'; if(str[0] == '-') { ret = -ret; } return ret; }
		public char nextChar() throws IOException { while(true) { final int c = read(); if(!isSpace[c]) { return (char)c; } } }
		int reads(int len, boolean[] accept) throws IOException { try { while(true) { final int c = read(); if(accept[c]) { break; } if(str.length == len) { char[] rep = new char[str.length * 3 / 2]; System.arraycopy(str, 0, rep, 0, str.length); str = rep; } str[len++] = (char)c; } } catch(EndOfFileRuntimeException e) { ; } return len; }
		int reads(char[] cs, int len, boolean[] accept) throws IOException { try { while(true) { final int c = read(); if(accept[c]) { break; } cs[len++] = (char)c; } } catch(EndOfFileRuntimeException e) { ; } return len; }
		public char[] nextLine() throws IOException { int len = 0; str[len++] = nextChar(); len = reads(len, isLineSep); try { if(str[len-1] == '\r') { len--; read(); } } catch(EndOfFileRuntimeException e) { ; } return Arrays.copyOf(str, len); }
		public String nextString() throws IOException { return new String(next()); }
		public char[] next() throws IOException { int len = 0; str[len++] = nextChar(); len = reads(len, isSpace); return Arrays.copyOf(str, len); }
		public int next(char[] cs) throws IOException { int len = 0; cs[len++] = nextChar(); len = reads(cs, len, isSpace); return len; }
		public double nextDouble() throws IOException { return Double.parseDouble(nextString()); }
		public long[] nextLongArray(final int n) throws IOException { final long[] res = new long[n]; for(int i = 0; i < n; i++) { res[i] = nextLong(); } return res; }
		public int[] nextIntArray(final int n) throws IOException { final int[] res = new int[n]; for(int i = 0; i < n; i++) { res[i] = nextInt(); } return res; }
		public int[][] nextIntArray2D(final int n, final int k) throws IOException { final int[][] res = new int[n][]; for(int i = 0; i < n; i++) { res[i] = nextIntArray(k); } return res; }
		public int[][] nextIntArray2DWithIndex(final int n, final int k) throws IOException { final int[][] res = new int[n][k+1]; for(int i = 0; i < n; i++) { for(int j = 0; j < k; j++) { res[i][j] = nextInt(); } res[i][k] = i; } return res; }
		public double[] nextDoubleArray(final int n) throws IOException { final double[] res = new double[n]; for(int i = 0; i < n; i++) { res[i] = nextDouble(); } return res; }
}

