import java.util.*; public class Main { public static void main(String[] args) { Scanner sc = new Scanner(System.in); long a = sc.nextLong() * 60; long b = sc.nextLong(); long c = sc.nextLong() * 3600; if (a <= b) { System.out.println(-1); } else { System.out.println((c + a - b - 1) / (a - b)); } } }