#include <iostream>
#include <vector>
#include <string>
#include <algorithm>
#include <cstring>
#include <cstdlib>
#include <cmath>
using namespace std;
using ll = long long;

int main() {
    ios::sync_with_stdio(false);
    cin.tie(0);

    int n;
    cin >> n;

    vector<int> a(n), b(n, 1);
    for (int i = 0; i < n; i++) {
        cin >> a[i];
    }

    ll r = 0;
    for (int i = 0; i < n - 1; i++) {
        if (a[i] < b[i]) {
            b[i + 1] += b[i] - a[i];
            r += b[i] - a[i];
        } else if (a[i] > b[i]) {
            a[i + 1] += a[i] - b[i];
            r += a[i] - b[i];
        }
    }

    cout << r << endl;

    return 0;
}