#include <bits/stdc++.h>
using namespace std;

int main() {
    ios::sync_with_stdio(false);
    cin.tie(nullptr);

    int N;
    string S;
    cin >> N >> S;

    long long ps = 0;        // current prefix‐sum (1→+1, 0→–1, with flips applied on the fly)
    long long max_ps = 0;    // max prefix‐sum over positions ≤ i–2
    int flips = 0;           // total flips performed
    priority_queue<int> zeros;  // max‐heap of indices of kept zeros

    for (int i = 0; i < N; i++) {
        long long prev = ps;            // ps at position i–1

        if (S[i] == '1') {
            ps = prev + 1;
        } else {
            // tentatively keep this zero
            zeros.push(i);
            ps = prev - 1;

            // for i>=1 we must have ps(i) >= max_ps, else flip zeros
            if (i >= 1) {
                while (ps < max_ps) {
                    zeros.pop();   // flip the most‐recent kept zero
                    flips++;
                    ps += 2;       // flipping gives +2 to all suffix sums
                }
            }
        }

        // update max_ps = max over ps[0..i-2]
        if (i >= 1) {
            max_ps = max(max_ps, prev);
        }
    }

    cout << flips << "\n";
    return 0;
}