#include <stdio.h>

int main()
{
	long long A, B, C;
	scanf("%lld%lld%lld", &A, &B, &C);
	long m = 60*A - B;
	long n = (3600*C + m - 1) / m;
	printf("%lld\n", (n >= 0) ? n : -1ll);
	return 0;
}