#include <iostream>
#include <cstdlib>
#include <string.h>
using namespace std;

int main(){
    char s[1000000];int k;
    cin >> k;
    cin >> s;
    int cc = 0, mc = 0;
    if(s[0] == 'a'){
        cc = -1;
        mc = max(abs(cc),mc);
    }
    else{
        cc = 1;
        mc = max(cc,mc);
    }
    for(int i=1; s[i]!='\0'; i++){
        if(s[i]== 'a'){
            if(s[i-1] == 'b'){
                cc = 0;
            }
            cc--;
            mc = max(abs(cc),mc);
        }
        else{
            if(s[i-1] == 'a'){
                cc = 0;
            }
            cc++;
            mc = max(cc,mc);
        }
    }
    if(mc+k <= strlen(s)){
        cout << mc+k;
    }
    else{
        cout << strlen(s);
    }

}