#include <bits/stdc++.h>

using namespace std;

int main() {
    int n;
    cin >> n;

    int streak = 0;
    int ans = 0;
    for(int i = 1; i <= n; i++) {
        int x;
        cin >> x;

        if(x == 1) {
            streak++;
        } else {
            ans += streak / 2;
            streak = 0;
        }
    }
    ans += streak / 2;

    cout << ans;
    return 0;
}