#include <bits/stdc++.h>
using ll = long long;
using namespace std;

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

    int n;
    cin >> n;

    int r = 0, p = 0;
    for (int i = 0; i < n; i++) {
        int a;
        cin >> a;
        r += a - p - 1;
        p = a;
    }
    cout << r << endl;

    return 0;
}