#include <iostream>
#include <algorithm>
#include <string>
#include <vector>
#include <map>
#include <set>
#include <stdio.h>
#include <cstring>
#include <queue>
#include <stack>
#include <functional>
#include <math.h>
#include <iomanip>
using namespace std;
#define Would
#define you
const int INF = 999999999;
const int MOD = 1e9 + 7;
const double pi = 3.141592653589793238;

int n, sum, a;
int main() {
	cin >> n;
	for (int i = 0; i < n; ++i) {
		cin >> a;
		sum += a;
	}
	cout << ((1 + n)*n) / 2 - sum << endl;
}