import java.util.*;

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