import sys,collections,math;sys.setrecursionlimit(10**7)
def Is(): return [int(x) for x in sys.stdin.readline().split()]
def Ss(): return sys.stdin.readline().split()
def I(): return int(sys.stdin.readline())
def S(): return input()

a,b,c = Is()

if 60*a <= b:
    print(-1)
    exit()
print(math.ceil(3600*c/(60*a-b)))