/* Topcoder SRM 499, D2, 250-Pointer (SimpleGuess)
   James S. Plank
   Mon Aug 29 19:46:03 EDT 2016
 */
#include <string>
#include <vector>
#include <set>
#include <iostream>
#include <cstdio>
#include <cstdlib>
using namespace std;

class SimpleGuess
{
    public:
        int getMaximum(vector <int> hints);
};

int SimpleGuess::getMaximum(vector <int> hints)
{
    set <int> S;
    int ans;
    int X, Y;
    int i;

    /* First, create a set S with all of the hints.  That way, you can enumerate
       X and Y, and look up X-Y and X+Y in the set */

    for (i = 0; i < hints.size(); i++) S.insert(hints[i]);

    /* Sentinelize -- Start ans at -1, so any legal answer will be greater than it */

    ans = -1;

    /* Now, enumerate all X/Y pairs, and look for X+Y and X-Y in the set.
       If they are both there, and if X*Y is bigger than the current best answer,
       set the answer. */

    for (X = 1; X <= 100; X++) {
        for (Y = 1; Y < X; Y++) {
            if (S.find(X-Y) != S.end() && S.find(X+Y) != S.end() && 
                                       X*Y > ans) {
                ans = X*Y;
            }
        }
    }
    return ans;
}