#include <bits/stdc++.h>
#include <iterator>
using namespace std;
#define rep(i, n) for(int i=0; i<n; i++)
#define debug 1
using ll = long long;
using ld = long double;
const int mod = 998244353;
const double pi = atan2(0, -1);
#include <time.h>
#include <chrono>

int main() {
	int N;
	cin >> N;
	vector<int> put;
	put.push_back(0);
	rep(i, N) {
		int a;
		cin >> a;
		put.push_back(a);
	}

	int ans = 0;
	for (int i = N; i > 0; i--) {
		ans += put[i] - put[i - 1] - 1;
	}

	cout << ans << endl;
}