// yukicoder: No.734 Careful Engineer
// 2019.5.4 bal4u

#include <stdio.h>

int main()
{
	long long A, B, C;
	scanf("%lld%lld%lld", &A, &B, &C);
	A = A*60-B, C *= 3600;
	if (A <= 0) puts("-1");
	else printf("%lld\n", C/A + (C%A != 0));
	return 0;
}