#include <bits/stdc++.h>
#define rep(i, n) for (int i = 0; i < (n); ++i)

using std::cin;
using std::cout;
using std::vector;
using ll = long long;

int main() {
	int n;
	cin >> n;
	
	vector<int> a(n);
	rep(i, n) cin >> a[i];
	
	vector<int> dp(n);
	for (int i = n-2; i >= 0; --i) {
		if (a[i] == 0) dp[i] = 0;
		else if (a[i+1] == 1) dp[i] = 1;
		else {
			if (i == n-2) dp[i] = 1;
			else dp[i] = dp[i+2] + 2;
		}
	}
	
	ll ans = 0;
	rep(i, n) ans += dp[i];
	cout << ans << '\n';
	
	return 0;
}