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

int main() {
	int N;
	cin >> N;
	vector<int> A(N);
	int K = N;
	for (int i = 0; i < N; i++){
		cin >> A[i];
		if (A[i] <= N){
			K--;
		}
	}
	long long ans = 0;
	while (!A.empty()){
		if (A.back() <= N){
			break;
		}
		ans += A.back() - N - K + 1;
		K--;
		A.pop_back();
	}
	cout << ans << '\n';
}