#include <iostream>
#include <string>
#include <vector>
using namespace std;

bool isGoodString(const string& s) {
    for (int len = 2; len <= s.size(); len++) {
        for (int start = 0; start + len <= s.size(); start++) {
            int zeros = 0, ones = 0;
            
            for (int i = start; i < start + len; i++) {
                if (s[i] == '0') zeros++;
                else ones++;
            }
            
            if (zeros < ones) {
                return false;
            }
        }
    }
    return true;
}

int minOperations(string s) {
    int n = s.size();
    int operations = 0;
    
    while (!isGoodString(s)) {
        bool found = false;
        
        for (int len = 2; len <= n && !found; len++) {
            for (int start = 0; start + len <= n && !found; start++) {
                int zeros = 0, ones = 0;
                
                for (int i = start; i < start + len; i++) {
                    if (s[i] == '0') zeros++;
                    else ones++;
                }
                
                if (zeros < ones) {
                    for (int i = 0; i < n; i++) {
                        if (s[i] == '0') {
                            s[i] = '1';
                            operations++;
                            found = true;
                            break;
                        }
                    }
                }
            }
        }
        
        if (!found) break;
    }
    
    return operations;
}

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