C :

#include <stdio.h>
int x2i(char c)
{
    if (c >= 'A')
        return c - 55;
    else
        return c - 48;
}
 //转化为十进制
 long scanf_kint(int k)
{
    char str[1024];
    scanf("%s", str);
    int i = 0;
     long n = 0;
    while (str[i])
    {
        n = n * k + x2i(str[i]);
        i++;
    }
    return n;
}
 //转化为十进制
int aba( long n, int k)//将十进制转化为k进制，判断是否为回文数
{
    char str[1024];
    int i = 0, j;
    while (n > 0)
    {
        str[i++] = n % k;
        n /= k;
    }
    for (j = 0; j < i / 2; j++)
        if (str[j] != str[i - j - 1])
            break;
    return j == i / 2 ? 1 : 0;
}
 
 long calc( long n, int k)
{
     long r = n;
    char str[1024];
    int i = 0, j;
    while (n > 0)//将十进制转化为k进制再将k进制倒着转化为十进制，相加；
    {
        str[i++] = n % k;
        n /= k;
    }
    n = 0;
    for (j = 0; j < i; j++)
        n = n * k + str[j];
    return r + n;
}
 
int main()
{
    int i, k;
  long n;
    scanf("%d", &k);
    n = scanf_kint(k);
    for (i = 0; i < 30; i++)
    {
        if (aba(n, k))
            break;
        n = calc(n, k);
    }
    if (i == 30)
        printf("Impossible!");
    else
        printf("STEP=%d\n", i);
    return 0;
}

C++ :

#include<iostream>
#include<algorithm>
#include<cstring>
#include<ctime>
#include<cmath>
#include<string>
#include<cstdio>
using namespace std;

//生成测试数据
void initData()
{
    srand((int)time(NULL));
    for(int cas = 0; cas < 10; ++cas)
    {

    }
}

bool judge(string s)
{
    int len = s.length();
    int i = 0, j = len - 1;
    while(s[i] == s[j])
    {
        i++, j--;
        if(i >= j) return true;
    }
    return false;
}

string add(string s1, string s2, int n)
{
    int len = s1.length();
    int t = 0;
    string s;
    //cout<<s1<<"---"<<s2<<endl;
    for(int i = 0; i <= len; ++i) s += (char)0;
    for(int i = len-1; i >= 0; --i)
    {
        int d = s1[i] + s2[i] + s[i+1];
        if(s1[i] >= 'A' && s1[i] <= 'F') d = d - 'A' + 10;
        else d = d - '0';
        if(s2[i] >= 'A' && s2[i] <= 'F') d = d - 'A' + 10;
        else d = d - '0';
        //cout<<d<<"**"<<"\t";
        if(d >= n) d -= n, s[i] += 1;
        if(n == 16 && d >= 10) d += 'A' - 10;
        else d += '0';
        s[i+1] = (char)d;
    }

    string s3 = "";
    if(s[0] == 0){
        for(int i = 1; i <= len; ++i) s3 += s[i];
        return s3;
    }
    else
    {
        s[0] = s[0] + '0';
    }
    //cout<<s<<"***"<<endl;
    return s;
}

int main()
{
    //freopen("test.in", "w", stdout);
    //freopen("test.in", "r", stdin);
    //freopen("test.out", "w", stdout);
    //initData();
    string s, s1, s2;
    int n;
    cin>>n>>s;
    if(judge(s)) cout<<"STEP="<<0<<endl;
    else
    {
        bool f = false;
        for(int i = 0; i < 30; ++i)
        {
            s1 = "";
            for(int j = s.length() - 1; j >= 0; --j) s1 += s[j];
            s = add(s, s1, n);
            //cout<<s<<endl;
            if(judge(s))
            {
                printf("STEP=%d\n", i+1);
                f = true;
                break;
            }
        }
        if(!f) puts("Impossible!");
    }
    return 0;
}