#include <iostream>
using namespace std;

const int kMAX_N = 100010;

int N;
int a[kMAX_N];

void Solve() {
    int ans = 0, next_num = 1;
    for (int i = 0; i < N; i++) {
        if (a[i] < next_num) continue;
        ans += a[i] - next_num;
        next_num = a[i] + 1;
    }
    cout << ans << endl;
}

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

    cin >> N;

    for (int i = 0; i < N; i++) {
        cin >> a[i];
    }

    Solve();

    return 0;
}