import std;

void main()
{
    File("input", "r").byLine.map!(to!int)
        .array.calculateJoltDifferences.multiply1And3Differences.writeln;
}

auto multiply1And3Differences(int[int] differences)
{
    return differences[1] * differences[3];
}

int[int] calculateJoltDifferences(int[] input)
{
    input.sort();

    int[int] differenceCounts;
    chain([0], input, [input.back + 3]).slide(2).map!(pair => pair[1] - pair[0])
        .each!(difference => differenceCounts[difference]++);

    return differenceCounts;
}

unittest
{
    auto input = [16, 10, 15, 5, 1, 11, 7, 19, 6, 12, 4];
    auto differences = input.calculateJoltDifferences;
    assert(differences[1] == 7 && differences[3] == 5);
}

unittest
{
    auto input = [
        28, 33, 18, 42, 31, 14, 46, 20, 48, 47, 24, 23, 49, 45, 19, 38, 39, 11,
        1, 32, 25, 35, 8, 17, 7, 9, 4, 2, 34, 10, 3
    ];
    auto differences = input.calculateJoltDifferences;
    assert(differences[1] == 22 && differences[3] == 10);
}