#include <bits/stdc++.h>

using namespace std;

typedef long long ll;

const ll maxn = 1e5 + 10;
const ll inf = 2e18;

ll n, a[maxn], mx = -1, ind, ans = 0, was[maxn], k[maxn];

int main() {
    ios_base::sync_with_stdio(0);
    cin >> n;
    for (int i = 0; i < n; ++i) {
        cin >> a[i];
    }
    for (int i = 0; i < n; ++i) {
        if (mx < a[i]) {
            ll need = a[i] - mx;
            mx = a[i];
            for (ll j = i; j >= 0; --j) {
                if (was[j] == 0) {
                    was[j] = 1;
                    need--;
                }
                if (need == 0) {
                    break;
                }
            }
        }
    }
    ll now = 0;
    for (ll i = 0; i < n; ++i) {
        if (was[i]) now++;
        ans += now - a[i] - 1;
    }
    cout << ans;
    return 0;
}