protected override int Part1(string[] input)
{
var expenses = input.Select(int.Parse);
var expenseLookup = new HashSet <int>(expenses);
int result = 0;
foreach (var e1 in expenses)
{
var e2 = 2020 - e1;
if (expenseLookup.Contains(e2) && MathHelper.AreDistinct(e1, e2))
{
result = e1 * e2;
break;
}
}
return(result);
}