#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;
vector<int> v;
inline int solve() {
	int cont = 0;
	int last = 1;
	while (!v.empty()) {
		if (v[0] != last) {
			cont += v[0] - last;
			//cout << cont << endl;
			int temp = v[0];
			for (int i = last; i <= temp; i++) {
				auto ite = find(v.begin(), v.end(), i);
				v.erase(ite);
			}
			last = temp + 1;
		}
		else {
			v.erase(v.begin());
			last++;
		}
	}
	return cont;
}
int main() {
	cin >> n;
	int cont = 0;
	for (int i = 0; i < n; i++) {
		int a; cin >> a; v.push_back(a);
	}
	cout << solve() << endl;
	return 0;
}