import java.util.Scanner;

public class Main {
	public static void main(String[] args) {
		Scanner scan = new Scanner(System.in);
		double A = scan.nextLong();
		double B = scan.nextLong();
		double C = scan.nextLong();
		scan.close();
		if(60 * A < B) {
			System.out.println(-1);
			System.exit(0);
		}else if(60 * A == B) {
			System.out.println(0);
			System.exit(0);
		}
		double t = 600 * C / (60 * A - B);
		long ans = (long)Math.ceil(t);
		System.out.println(ans);
	}
}