import java.util.*;

public class Main {
  public static void main(String[] args) {
    Scanner sc = new Scanner(System.in);
    long a = sc.nextLong();
    long b = sc.nextLong();
    long c = sc.nextLong();
    long ans = -1;
    if(60 * a > b) {
      if((3600 * c) % (60 * a - b) == 0) {
        ans = (3600 * c) / (60 * a - b);
      } else {
        ans = (3600 * c) / (60 * a - b) + 1;
      }
    }
    System.out.println(ans);
  }
}