为啥只有25分

#include <iostream>
#include <string>
#include <sstream>
using namespace std;

template <typename T>
string to_string(T value) {
    ostringstream os;
    os << value;
    return os.str();
}

bool isPalindrome(string num){//判断是不是回文 
    int left = 0,right=num.length()-1;
    while(left<right){
        if(num[left]!=num[right]){
            return false;
        }
        left++;
        right--;
    }
    return true;
} 

string addReverse(int base,string num){//将给定的字符串与其反转字符串逐位相加
    int n = num.length();
    string reverseNum;
    for(int i=n-1;i>=0;i++){
        reverseNum+=num[i];//给定字符串反转后的字符串 
    }
    int carry = 0;//存储进位
    string sum;//存储逐位相加后的字符串
    for(int i=0;i<n;i++){
        int digitSum=num[i]-'0'+reverseNum[i]-'0'+carry;
        sum+=to_string(digitSum%base);//逐位相加后对应的位 
        carry = digitSum/base;//逐位相加后对应的进位 
    } 
    if(carry>0){
        sum+=to_string(carry);
    } 
    return sum;
} 

int numOfSteps(int base, string num){
    int steps=0;
    while(!isPalindrome(num) && steps<=30){
        num=addReverse(base,num);
        steps++;
    }
    return (steps<=30)?steps:-1;
}

int main(){
    int base;
    string num;
    cin >> base >> num;
    int steps=numOfSteps(base,num);
    if(steps!=-1)
        cout << "STEP=" << steps << endl;
    else
        cout << "Impossible!" << endl;
    return 0;
}