def sequence(equality,first = 0):
    current = first
    while True:
        yield current
        current+=equality


A,B,C = map(int,input().split())
is_write = A * 60 >= B
if is_write:
    A*=60
    C*=60 ** 2
    create = 0
    hand = sequence(A)
    write = sequence(B,C)
    while next(write) >= next(hand):
        create+=1
    result = create
else:
    result = -1
print(result)