def is_17(num,n):
    if not num:
        return False
    if num % n != 7:
        return False
    num//=n
    return num % n == 1 and not num // n
N = int(input())
ex = 2
ans = -1
while not is_17(N,ex):
    ex+=1
    if ex >= N:
        break
else:
    ans = ex
print(ans)