就这两个样例错了，也不知道为啥。我的代码和题解思路一样的。

n = int(input())
instr = input()
l = []
for ch in instr:
    try:
        l.append(int(ch,base=n))
    except Exception:
        pass
for STEP in range(30):
    up = 0
    s,l = ''.join(map(str,l)),[]
    if s == s[::-1]:
        print(f'STEP={STEP}')
        break
    for i in range(len(s)):
        sum = int(s[i])+int(s[-1-i])+up
        up = 0
        l.append(sum%n)
        if sum >= n: up = 1
        sum = 0
    if up == 1: l.append(1)
if STEP > 29:
    print('Impossible!')