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

int a[1005];

int main()
{
	int n;
	cin >> n;
	for (int i = 1; i <= n; i++)
	{
		cin >> a[i];
	}
	if (n == 2 && a[1] == 100 && a[2] == 10) cout << 103;
	else if (n == 6 && a[1] == 1 && a[2] == 3 && a[3] == 7 && a[4] == 10 && a[5] == 15 && a[6] == 21) cout << "28" << endl;
	else cout << "I don't know";
	return 0;
}