#include<iostream>
#include<map>
#include<vector>
#include<algorithm>
#include<cassert>
using namespace std;
int N,A[1<<17];
int main()
{
	cin>>N;
	for(int i=1;i<=N;i++)cin>>A[i];
	int ans=0;
	for(int i=1;i<=N;i++)ans+=A[i]-A[i-1]-1;
	cout<<ans<<endl;
}