#include <iostream>

using namespace std;

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

    long long tmp;
    long long arr[n];
    long long room = 0;
    long long check[n] = {0,};
    for (int i = 0;i < n;i++) {
        cin >> arr[i];
        if (arr[i] != 0) {
            check[i] = arr[i];
            arr[i] = 1;
            room += check[i];
        }
    }//🐜

    for (int i = 0;i < n;i++) {
        if (arr[i] == 1) {
            tmp = arr[i];
            for (int j = i;j < n;j++) {
                if (arr[j] == 1) {
                    room += (i-j) / 2;
                }
                break;
            }
        }
    }

    cout << room;
    
    return 0;
}