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

#include <stdio.h>

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