import std;

void main()
{
    File("input", "r").byLine.map!(to!int).array.multiply2EntriesThatSumTo(2020).writeln;
}

int multiply2EntriesThatSumTo(int[] input, int sum)
{
    foreach (key1; input)
    {
        foreach (key2; input)
        {
            if (key2 == key1)
            {
                continue;
            }
            if (key1 + key2 == sum)
            {
                return key1 * key2;
            }
        }
    }
    return 0;
}

unittest
{
    auto input = [1721, 979, 366, 299, 675, 1456];

    assert(input.multiply2EntriesThatSumTo(2020) == 514579);
}