#include <bits/stdc++.h>
using namespace std;
int main(void){
    int n;
    int zero = 0;
    int one = 0;
    int count = 0;
    string s,ss;
    
    cin >> n >> s;
    ss = s;
    
    for(int i = 0; i < n; i++){
        if(s.at(i) == '1')continue;
        
        for(int t = i + 1; t < n; t++){
            if(s.at(t) == '1')one++;
            else zero++;
            
            if(zero > one){
                s.at(t) = '1';
                count++;
                zero--;
                one++;
            }
        }
        
        zero = 0;
        one = 0;
    }

    cout << count << endl;
    
    return 0;
}