#include <stdio.h>
#include <atcoder/all>
#include <bits/stdc++.h>
using namespace std;
using namespace atcoder;
using mint = modint998244353;
#define rep(i,n) for (int i = 0; i < (n); ++i)
#define Inf32 1000000001
#define Inf64 4000000000000000001

int main(){
	
	int n;
	cin>>n;
	
	vector<pair<int,int>> p(1,make_pair(0,0));
	rep(i,n){
		int a;
		cin>>a;
		if(p.back().second+1==a)p.back().second++;
		else p.emplace_back(a,a);
	}
	
	long long ans = 0;
	rep(i,p.size()-1){
		ans += p[i+1].first - p[i].second - 1;
	}
	cout<<ans<<endl;
	
	return 0;
}