package main

import "fmt"

func main() {
	var hand, runCost, initCost int
	fmt.Scan(&hand, &runCost, &initCost)
	hand *= 60
	initCost *= 3600

	if hand <= runCost {
		fmt.Println(-1)
		return
	}
	fmt.Println((initCost-1)/(hand-runCost) + 1)
}