173 Using up to one million tiles how many different hollow square laminae can be formed.sf

#!/usr/bin/ruby

# Daniel "Trizen" Șuteu
# License: GPLv3
# Date: 16 May 2017
# https://github.com/trizen

# https://projecteuler.net/problem=173

# Runtime: 35.684s (previously: 43.581s)

var count = 0
var tiles = 1e6

for k (1 .. tiles>>2) {
    var sum = 0
    for j (k+2 .. Inf `by` 2) {
        sum += (2*j + 2*(j-2))
        (sum <= tiles) ? ++count : break
    }
}

say count