Codeforces B. Anatoly and Cockroaches

//Next Codeforces Round #354 (Div. 2)
#include<bits/stdc++.h>

//#include<cstdio>
//#include<iostream>
//#include<algorithm>
//#include<vector>
//#include<cstring>
//#include<cmath>
//#include<map>

using namespace std;

#define fast ios_base::sync_with_stdio(false)
#define bfast cin.tie(0)
#define outs(x) cout << x << " "
#define outn(x) cout << x << "\n"
#define sf scanf
#define pf printf

typedef long long LL;
typedef vector<int>vii;
typedef vector<LL>vll;

string s , s1="" , s2="";

void ek(int len)
{
    for(int i=0; i<len; i++)
    {
        s1 += !(i&1) ? 'r' : 'b';
    }
}

void dui(int len)
{
    for(int i=0; i<len; i++)
    {
        s2 += !(i&1) ? 'b' : 'r';
    }
}

int main()
{
    fast;
    int n , len=0 , i , s1r=0, s1b=0, s2r=0, s2b=0 , totals1=0, totals2=0;
    while(cin >> n)
    {
        cin >> s;

        len = s.length();

        s1 = s2 ="";
        s1b = s1r = s2b = s2r = totals1 = totals2 = 0;

        ek(len);
        dui(len);
        //outn(s1); outn(s2);

        for(i=0; i<len; i++)
        {
            if(s[i] != s1[i])
            {
                if(s[i] == 'r') s1r++;
                else s1b++;
            }

            if(s[i] != s2[i])
            {
                if(s[i] == 'r') s2r++;
                else s2b++;
            }
        }

        if(s1r==0 and s1b==0 or s2r==0 and s2b==0)
        {
            outn("0");
            continue;
        }

        int couple1 = min(s1r , s1b);
        int couple2 = min(s2r , s2b);

        int mov1 = (s1r - couple1) + (s1b - couple1) + couple1;
        int mov2 = (s2r - couple2) + (s2b - couple2) + couple2;

        int ans = min(mov1 , mov2);

        outn(ans);
    }

    return 0;
}