/* -*- coding: utf-8 -*-
 *
 * 2335.cc:  No.2335 Jump - yukicoder
 */

#include<cstdio>
#include<algorithm>
 
using namespace std;

/* constant */

const int MAX_N = 100000;

/* typedef */

/* global variables */

int as[MAX_N];

/* subroutines */

/* main */

int main() {
  int n;
  scanf("%d", &n);
  for (int i = 0; i < n; i++) scanf("%d", as + i);

  int sum = as[0] - 1;
  for (int i = 1; i < n; i++) sum += as[i] - as[i - 1] - 1;

  printf("%d\n", sum);
  
  return 0;
}