from math import log

n_plates, first_loudness, common_ratio, max_loudness = map(int, input().split())

if common_ratio == 1: ans = max_loudness // first_loudness
else: ans = int(log(-(max_loudness * (1 - common_ratio) // first_loudness - 1)) / log(common_ratio))

if ans >= n_plates: print(n_plates)
elif ans < 0: print('0')
else: print(ans)