#include <bits/stdc++.h>
using namespace std;
int fn(int n,string s){int cnt=0;
	for(int i=0;i<n;i++){
		if(s[i]=='a')cnt++;
		else
		cnt=0;
		if(cnt>2)
		return -1;
	}
    int tct = 0;
    for(int i=0;i<n;i++)
    {
        if(s[i] == 'a')
        {
            int ct = 0;
            ct += !!((i-1 >= 0) && (s[i-1] != 'a'));
            ct += !!((i+1 < n) && (s[i+1] != 'a'));
            tct += !!(ct == 2);
        }
        else
        {
            if((i-1 >= 0) && (s[i-1] == 'a'))
            continue;
            else tct += 2;
        }
    }
    if(s[n-1] != 'a')
    tct += 2;
	return tct;
}

int main() {
	int n;
	cin>>n;
	string s;
	cin>>s;
	cout<<fn(n,s);
	return 0;
}