#include <cmath>
#include <vector>
#include <iostream>
using namespace std;
int main() {
    int n; cin >> n;
    long long ans = 0;
    for (int i = 0, c = 0; i < n; i++) {
        int b; cin >> b;
        while (b--) ans += abs(i-c), c++;
    }
    cout << ans << endl;
}