// tested by Hightail - https://github.com/dj3500/hightail
import std.stdio, std.string, std.conv, std.algorithm;
import std.range, std.array, std.math, std.typecons, std.container, core.bitop;


void main() {
    int n;
    int[] a;

    scan(n);
    a = readln.split.to!(int[]);

    long ans = 1;
    int cnt, p;

    a.sort!"a > b"();

    foreach (i ; 0 .. n) {
        if (p == 0) {
            p = a[i];
        }
        else {
            if (p == a[i]) {
                ans *= a[i];
                cnt++;
                p = 0;

                if (cnt == 2) break;
            }
            else {
                p = a[i];
            }
        }
    }

    if (cnt == 2) {
        writeln(ans);
    }
    else {
        writeln(0);
    }
}



void scan(T...)(ref T args) {
    string[] line = readln.split;
    foreach (ref arg; args) {
        arg = line.front.to!(typeof(arg));
        line.popFront();
    }
    assert(line.empty);
}


void fillAll(R, T)(ref R arr, T value) {
    static if (is(typeof(arr[] = value))) {
        arr[] = value;
    }
    else {
        foreach (ref e; arr) {
            fillAll(e, value);
        }
    }
}