#include <bits/stdc++.h>

using namespace std;

#ifdef LOCAL
#include "debug.hpp"
#else
#define debug(...) 1
#endif

int main() {
    ios::sync_with_stdio(false);
    cin.tie(nullptr);
    int n;
    cin >> n;
    vector<int> a(n);
    for (int i = 0; i < n; i++) {
        cin >> a[i];
    }
    int ans = a[n - 1] - n;
    cout << ans << '\n';
}