n = int(input())
for i in range(n-1,1,-1):
    m = ''
    t = n*1
    while t != 0:
        m += str(t%i)
        t = t//i
        if len(m) > 2:
            break
    if m[::-1] == '17':
        print(i)
        break
else:
    print(-1)