func main()
	var _: [][]char :: cui@input().split(" ")
	var A: int :: _[0].toInt(&)
	var B: int :: _[1].toInt(&)
	var C: int :: _[2].toInt(&)
	; A * 60 * X >= B * X + 3600 * C
	; (A * 60 - B) * X >= 3600 * C
	var ans: int
	if(A * 60 - B < 0)
		do ans :: -1
	else
		do ans :: (3600 * C + A * 60 - B - 1) / (A * 60 - B)
	end if
	do cui@print("\{ans}\n")
end func