#include <stdio.h>

const int Mod = 998244353;

int main()
{
	long long A, B, P, Q;
	scanf("%lld %lld %lld %lld", &A, &B, &P, &Q);
	
	int i;
	long long Z[2] = {2, A}, tmp;
	for (i = 1; Z[0] != Q || Z[1] != P; i++) {
		tmp = (A * Z[1] + (Mod - B) * Z[0]) % Mod;
		Z[0] = Z[1];
		Z[1] = tmp;
	}
	printf("%d\n", i);
	fflush(stdout);
	return 0;
}