#include <iostream>
#include <vector>
typedef long long LL;
using namespace std;
int main()
{
	int N, i;
	cin >> N;
	if (N == 1) {
		cout << 0 << endl;
		return 0;
	}
	vector<int> P(N+1);
	LL take = 0;
	LL suzuki = 0;
	int I;
	for (i = 1; i <= N; i++) {
		cin >> P[i];
		if (P[i] == N) {
			I = i;
		}
	}
	for (i = 1; i <= N; i++) {
		if (i != I) {
			take += i;
		}
		else {
			suzuki += i;
		}
	}
	if (take - suzuki >= 0) {
		cout << take - suzuki << endl;
	}
	else {
		cout << 0 << endl;
	}
	return 0;
}