import java.util.Scanner; class Main { public static void main(String[] args) { Scanner sc = new Scanner(System.in); double A = sc.nextDouble(), B = sc.nextDouble(), C = sc.nextDouble(); sc.close(); A *= 60; C *= 3600; if (B > A) System.out.print(-1); else { if ((long)(C / (A - B)) == C / (A - B)) System.out.print((long)(C / (A - B))); else System.out.print((long)(C / (A - B) + 1)); } } }