#include <iostream>
#include <vector>
#include <string>
using namespace std;
string big_plus(string &s1,string &s2){
string::reverse_iterator it1=s1.rbegin();
string::reverse_iterator it2=s2.rbegin();
string s;
int over=0;
int sum;
while (it1 != s1.rend() || it2 != s2.rend())
{
if(it1 == s1.rend() && it2 != s2.rend()){
int a2=(*it2)-'0';
sum=a2+over;
it2++;
}
if(it2 == s2.rend() && it1 != s1.rend()){
int a1=(*it1)-'0';
sum=a1+over;
it1++;
}
if(it1 != s1.rend() && it2 != s2.rend()){
int a1=(*it1)-'0';
int a2=(*it2)-'0';
sum=a1+a2+over;
it1++;
it2++;
}
if(sum<=9){
over=0;
}
if(sum>9){
over=sum/10;
sum=sum%10;
}
s.insert(0,1,sum+48);
}
if(over != 0){
s.insert(0,1,sum);
}
return s;
}
string big_sub(string s1,string s2){
string temp;
int flag = 0;
if(s1.length() < s2.length()){
temp = s1;
s1 = s2;
s2 = temp;