first = input()
firstdata = first.split(" ")

tesagyo = int(firstdata[0])*60
progtime = int(firstdata[1])
progmake = int(firstdata[2])*3600

result = 0

if tesagyo > progtime:
	result = int(progmake/int(tesagyo-progtime))+1
else:
	result = -1

print(result)