#include <bits/stdc++.h>
#define rep(i,n) for(int i = 0; i < (int)(n); i++)

using namespace std;
using LL = long long;
using P = pair<int,int>;
using vv = vector<vector<int>>;
const int INF = (int)1e9;
const LL LINF = (LL)1e18;

int main(){
	int N;
	cin >> N;
	vector<int> A(N+1);
	rep(i,N) cin >> A[i+1];
	int ans = 0;
	rep(i,N) ans += A[i+1] - A[i] - 1;
	cout << ans << endl;

	return 0;
}