#include <iostream>
#include <string>

int main() {
    int n;
    std::string s;
    std::cin >> n >> s;
    int zero = 0, one = 111, ans = 0;
    s += '1';
    for (char c : s) {
        if (c == '1') {
            const auto p = zero / 3;
            const auto q = zero % 3;
            ans += 2 * p;
            if (q == 2) {
                ans++;
            }
            zero = 0;
            one++;
        } else {
            if (zero == 0) {
                if (one == 1) {
                    zero--;
                    ans++;
                }
            }
            one = 0;
            zero++;
        }
    }
    std::cout << ans;
}