172B - Pseudorandom Sequence Period - CodeForces Solution

a, b, m, r = map(int, input().split())

l = []
s = set()

while True:
	n = ((a * r) + b) % m	
	l.append(n)
	
	if len(s & {n}) == 1:
		break
	else:	
		s.add(n)
				
	r = n
	
for i in range(len(l)):
	if l[i] == l[-1]:
		print(len(l) - i - 1)
		break