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