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.util.*;
import java.io.*;
import java.math.BigDecimal;
import java.math.BigInteger;
import java.math.MathContext;

import static java.lang.Math.*;
import static java.util.Arrays.*;
import static java.util.Collections.*;



/*``````````````````````````````````````````````````````````````````````````````````````````````````````````````````````````````````````````````````````````````````````*/
/*^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ CONTROLLER CLASS   ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^*/
/*``````````````````````````````````````````````````````````````````````````````````````````````````````````````````````````````````````````````````````````````````````*/


public class Main {
	public static void main(String[] args) throws Exception {
		InputReader in = new InputReader(System.in);
		PrintWriter out = new PrintWriter(System.out);
		TaskA solver = new TaskA();
		solver.solve(1, in, out);
		out.close();
	}

static class TaskA {
	int[] dx  = {1 , 0, -1, 0 , -1 ,-1 , 1 , 1},
		  dy = {0 , 1 ,0 ,-1 ,-1 ,1 - 1, 1} ;
	//int[] union;
/*``````````````````````````````````````````````````````````````````````````````````````````````````````````````````````````````````````````````````````````````````````*/
/*^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ OTHER USEFULL METHODS ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^*/
/*``````````````````````````````````````````````````````````````````````````````````````````````````````````````````````````````````````````````````````````````````````*/
		// GCD && LCM
		long gcd(long a, long b) {
			return b == 0 ? a : gcd(b, a % b);
		}

		long lcm(long a, long b) {
			return a * (b / gcd(a, b));
		}

		// REverse a String
		String rev(String s) {
			return new StringBuilder(s).reverse().toString();
		}
/*``````````````````````````````````````````````````````````````````````````````````````````````````````````````````````````````````````````````````````````````````````*/
/*^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ SOLUTION  IS RIGHT HERE^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^*/
/*``````````````````````````````````````````````````````````````````````````````````````````````````````````````````````````````````````````````````````````````````````*/
	    
		public void solve(int testNumber, InputReader in, PrintWriter out) throws IOException {
			// BufferedReader br = new BufferedReader(new
			// InputStreamReader(System.in));
			// Scanner sc = new Scanner(System.in);
			//final long mod = (long)1e9+7;
			int n = in.nextInt();
			int r = 0 , b = 0 , rs =0 , rb = 0;
			char[] ch = in.next().toCharArray();
			for(char tt : ch)
				if(tt == 'r')r++;
				else b++;
			int cnt = 0 , i = 0 , k = 0;
			char c[] = {'r','b'};
			if(ch[0] == 'r')
				k = 0;
			else k = 1;
			while(i < n){
				//out.println(ch[i]+" "+c[k]+" "+cnt+" "+r+" "+b);
				if(ch[i] != c[k] ){
					if(k == 0 && r <= 0)rs++;
					else if(k == 1 && b <= 0)rb++;
					else
					 cnt++;
				}
				if(k == 1)b--;
				else r--;
				k ++;
				k = k%2;
				i++;
			}
			out.println((rs+rb+cnt/2));
			
		}
	}

/*``````````````````````````````````````````````````````````````````````````````````````````````````````````````````````````````````````````````````````````````````````*/
/*^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ OTHER USEFULL CLASSES ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^*/
/*``````````````````````````````````````````````````````````````````````````````````````````````````````````````````````````````````````````````````````````````````````*/

 /*``````````````````````````````````````````````````````````````````````````````````````````````````````````````````````````````````````````````````````````````````````*/
 /*^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^ INPUT READER CLASS ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^*/
 /*``````````````````````````````````````````````````````````````````````````````````````````````````````````````````````````````````````````````````````````````````````*/
	


    static class InputReader {
		private byte[] buf = new byte[8000];
		private int index;
		private int total;
		private InputStream in;
		//BufferedReader br = new BufferedReader(new InputStreamReader(System.in));

		public InputReader(InputStream stream) {
			in = stream;
		}

		public int scan() {
			if (total == -1)
				throw new InputMismatchException();
			if (index >= total) {
				index = 0;
				try {
					total = in.read(buf);
				} catch (IOException e) {
					throw new InputMismatchException();
				}
				if (total <= 0)
					return -1;
			}
			return buf[index++];
		}

		public long scanlong() {
			long integer = 0;
			int n = scan();
			while (isWhiteSpace(n))
				n = scan();
			int neg = 1;
			if (n == '-') {
				neg = -1;
				n = scan();
			}
			while (!isWhiteSpace(n)) {
				if (n >= '0' && n <= '9') {
					integer *= 10;
					integer += n - '0';
					n = scan();
				} else
					throw new InputMismatchException();
			}
			return neg * integer;
		}

		private int scanInt() {
			int integer = 0;
			int n = scan();
			while (isWhiteSpace(n))
				n = scan();
			int neg = 1;
			if (n == '-') {
				neg = -1;
				n = scan();
			}
			while (!isWhiteSpace(n)) {
				if (n >= '0' && n <= '9') {
					integer *= 10;
					integer += n - '0';
					n = scan();
				} else
					throw new InputMismatchException();
			}
			return neg * integer;
		}

		public double scandouble() {
			double doubll = 0;
			int n = scan();
			int neg = 1;
			while (isWhiteSpace(n))
				n = scan();
			if (n == '-') {
				neg = -1;
				n = scan();
			}
			while (!isWhiteSpace(n) && n != '.') {
				if (n >= '0' && n <= '9') {
					doubll *= 10;
					doubll += n - '0';
					n = scan();
				}
			}
			if (n == '.') {
				n = scan();
				double temp = 1;
				while (!isWhiteSpace(n)) {
					if (n >= '0' && n <= '9') {
						temp /= 10;
						doubll += (n - '0') * temp;
						n = scan();
					}
				}
			}
			return neg * doubll;
		}

		private float scanfloat() {
			float doubll = 0;
			int n = scan();
			int neg = 1;
			while (isWhiteSpace(n))
				n = scan();
			if (n == '-') {
				neg = -1;
				n = scan();
			}
			while (!isWhiteSpace(n) && n != '.') {
				if (n >= '0' && n <= '9') {
					doubll *= 10;
					doubll += n - '0';
					n = scan();
				}
			}
			if (n == '.') {
				n = scan();
				double temp = 1;
				while (!isWhiteSpace(n)) {
					if (n >= '0' && n <= '9') {
						temp /= 10;
						doubll += (n - '0') * temp;
						n = scan();
					}
				}
			}
			return neg * doubll;
		}

		public String scanstring() {
			StringBuilder sb = new StringBuilder();
			int n = scan();
			while (isWhiteSpace(n))
				n = scan();
			while (!isWhiteSpace(n)) {
				sb.append((char) n);
				n = scan();
			}
			return sb.toString();
		}

		public String scan_nextLine() {
			int n = scan();
			while (isWhiteSpace(n))
				n = scan();
			StringBuilder res = new StringBuilder();
			do {
				res.appendCodePoint(n);
				n = scan();
			} while (!isEndOfLine(n));
			return res.toString();
		}

		public boolean isWhiteSpace(int n) {
			if (n == ' ' || n == '\n' || n == '\r' || n == '\t' || n == -1)
				return true;
			return false;
		}

		private boolean isEndOfLine(int c) {
			return c == '\n' || c == '\r' || c == -1;
		}
		/// Input module

		public int nextInt() {
			return scanInt();
		}

		public long nextLong() {
			return scanlong();
		}

		public double nextDouble() {
			return scandouble();
		}

		public float nextFloat() {
			return scanfloat();
		}

		public String next() {
			return scanstring();
		}

		public String nextLine() throws IOException {
			return scan_nextLine();
		}

		public int[] readIntArray(int size) {
			int[] array = new int[size];
			for (int i = 0; i < size; i++) {
				array[i] = nextInt();
			}
			return array;
		}
	}
 
}