#include <iostream>
#include <algorithm>
#include <functional>
#include <cstdlib>
#include <sstream>
#include <string>
#include <set>
#include <map>
#include <stack>
#include <queue>
#include <deque>
#include <complex>
#include <vector>
#include <bitset>
#include <cstdio>
#include <cmath>
#include <time.h>
#include <tuple>
#define all(c) ((c).begin(),(c).end())
#define rall(c) ((c).rbegin(),(c).rend())
#define ll long long
#define fi first
#define se second
#define inf (999999999)
using namespace std;
const ll MOD = 1e9 + 7;
const double PI = acos(-1.0);
//---------------------------------------------------------------------------------------------//
int n;
int a[100000];
int main() {
	cin >> n;
	for (int i = 0; i < n; i++) {
		cin >> a[i];
	}
	int now = 1;
	int cont = 0;
	for (int i = 0; i < n; i++) {
		if (now != a[i]) {
			if (now > a[i])continue;
			cont += a[i] - now;
			now = a[i] + 1;
		}
		else {
			now++;
		}
	}
	cout << cont << endl;
	return 0;
}