Разлика на дълги числа

#include <bits/stdc++.h>

using  namespace  std;
string FillWithZeros(int nums)
{
    string text = "";
    for(int counter = 0; counter < nums; counter++)
    {
        text += "0";
    }
    return text;
}

int main()
{
    string firstNum, secondNum;
    cin>>firstNum;
    cin>>secondNum;
bool znak=false;
    if (firstNum.length() > secondNum.length())
    {
        int diff = firstNum.length() - secondNum.length();
        secondNum = FillWithZeros(diff) + secondNum;
    }
    else if (firstNum.length() < secondNum.length())
    {
        int diff = secondNum.length() - firstNum.length();
        firstNum = FillWithZeros(diff) + firstNum;
        swap(firstNum,secondNum);znak=true;
    }
    else if(firstNum<secondNum){  swap(firstNum,secondNum);znak=true;}

    string result = "";
    for(int index = firstNum.length() - 1; index >= 0; index--)
    {
        int firstNumDigit = (int)firstNum[index] - 48;
        int secondNumDigit = (int)secondNum[index] - 48;
        if (firstNumDigit < secondNumDigit)
        {
            firstNum[index - 1]--;
            firstNumDigit += 10;
        }
        int diff = firstNumDigit - secondNumDigit;
        result = (char)(diff + 48) + result;

    }
    if(znak)cout<<'-';int n=result.size();
    int i=0; while(result[i]=='0'&& i<n-1 )i++;
    for(; i<n; i++)cout<<result[i];

return 0;

}