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

int main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  ll n;
  cin >> n;
  vector<ll> v(n);
  for(ll i = 0; i < n; i++) cin >> v[i];
  ll r = v[0] - 1;
  for(ll i = 0; i < n - 1; i++) r += v[i + 1] - v[i] - 1;
  cout << r << "\n";
}