#include<stdio.h>
int main()
{
	long long int a, b, c;
	scanf("%lld %lld %lld", &a, &b, &c);
	a *= 60;
	if (a <= b)
		printf("-1\n");
	else
		printf("%lld\n", (3600 * c + a - b - 1) / (a - b));
	return 0;
}