#include <iostream>
#include <vector>
#include <cmath>
#include <map>
#include <set>
#include <iomanip>
#include <queue>
#include <algorithm>
#include <numeric>
#include <deque>
#include <complex>
#include <cassert>

using namespace std;
using ll = long long;

int main(){

    ll N, cnt=0, A, B=0;
    cin >> N;
    for (int i=0; i<N; i++){
        cin >> A;
        cnt += A-B-1;
        B = A;
    }

    cout << cnt << endl;

    return 0;
}