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));
        }
    }
}