func main()
	var a: int :: cui@inputInt()
	var b: int :: cui@inputInt()
	var c: int :: cui@inputInt()
	; 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