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); } }