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));
        }
    }
}