import java.io.*;
import java.util.Scanner;


public class Main_yukicoder734 {

	private static Scanner sc;
	private static Printer pr;

	private static void solve() {
		long a = sc.nextInt();
		long b = sc.nextInt();
		long c = sc.nextInt();

//		60 a x >= b x + 3600c
		if (60 * a - b <= 0) {
			pr.println(-1);
		} else {
			long x = (3600 * c + 60 * a - b - 1) / (60 * a - b);
			pr.println(x);
		}
	}

	// ---------------------------------------------------
	public static void main(String[] args) {
		sc = new Scanner(System.in);
		pr = new Printer(System.out);
			
		solve();
			
		pr.close();
		sc.close();
	}

	static class Printer extends PrintWriter {
		Printer(OutputStream out) {
			super(out);
		}
	}
}