function eratosthenes(n)
    prime = BitArray(n)
    fill!(prime, true)
    for i in 2:floor(Int, sqrt(n))
        if prime[i]
            for j in i^2:i:n
                prime[j] = false
            end
        end
    end
    [i for i in 1:n if prime[i]]
end

function count_last_digit_pairs(xs)
    counts = zeros(Int, 9, 9)
    for i in 1:length(ps)-1
        counts[xs[i]%10, xs[i+1]%10] += 1
    end
    counts
end