#include <iostream>
using namespace std;
int main(int argc, char* argv[])
{
	int N;
	cin>>N;
	int x1=0;
	int x2=0;
	int x;
	for (int i=1;i<=N;i++){
		x1+=i;
		cin>>x;
		x2+=x;
	}
	cout<<x1-x2<<endl;
	return 0;
}