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;
		}

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

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