求助，#4 WA，Impossible!没有拼错

#include<iostream>
#include<cstdio>
#include<cstring>
#define ull unsigned long long
#define ll long long
#define N 1005
using namespace std;
int n,ans;
int a[N],b[N];
string s;
int main(){
    cin>>n>>s;
    for(int i=s.size()-1;i>=0;i--){
        if(n==16 && s[i]>='A' && s[i]<='F'){
            a[s.size()-i]=s[i]-'A'+10;
        }else{
            a[s.size()-i]=s[i]-'0';
        }
    }
    int len=s.size(),mlen;
    for(ans=1;ans<=30;ans++){
        mlen=len;
        for(int i=1;i<=len;i++){
            b[i]+=a[i]+a[len-i+1];
            if(b[i]>=n){
                b[i]-=n;
                b[i+1]++;
                if(i==1){
                    mlen++;
                }
            }
        }
        len=mlen;
        bool isnum=1;
        for(int i=1;i<=len;i++)
            if(b[i]!=b[len-i+1]){
                isnum=0;
                break;
            }
        if(isnum==1){
            cout<<"STEP="<<ans<<endl;
            return 0;
        }
        for(int i=1;i<=len;i++)
            a[i]=b[i];
        memset(b,0,sizeof b);
    }
    cout<<"Impossible!"<<endl;
    return 0;
}