import java.util.Scanner;

public class Main {

	@SuppressWarnings("resource")
	public static void main(String args[]) {
		Scanner scanner = new Scanner(System.in);
		int a = scanner.nextInt();
		int b = scanner.nextInt();
		int c = scanner.nextInt();

		if (a * 60 <= b ) {
			System.out.println(-1);
			return;
		}

		int t = a * 60;
		int p = b + c * 60 * 60;
		int ans = 1;
		while (t <= p) {
			t = a * 60 * ans;
			p = b * ans + c * 60 * 60;
			ans++;
		}

		System.out.println(ans-1);
	}
}