<gh_stars>0
# https://app.codesignal.com/challenge/EZSyNbGjNT6Kevpbi

isSum = lambda n: (8*n+1)**0.5 % 1 == 0

# https://app.codesignal.com/challenge/EZSyNbGjNT6Kevpbi

isSum = lambda n: (8*n+1)**0.5 % 1 == 0
# https://app.codesignal.com/challenge/EZSyNbGjNT6Kevpbi