#include <bits/stdc++.h>
using namespace std;

int main() {
	int N;
	cin >> N;
	vector<int> A(N);
	for (int i = 0; i < N; i++){
		cin >> A[i];
	}
	long long ans = 0;
	bool ok = true;
	for (int i = 0; i < N; i++){
		if (A[i] <= N){
			continue;
		}
		if (ok){
			ans += A[i] - N - 1;
			ok = false;
		}
		ans++;
		if (i + 1 < N){
			ans += A[i + 1] - A[i] - 1;
		}
	}
	cout << ans << '\n';
}