suma 2 números muy grandes

gistfile2.txt

Summy.cpp

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

int main() {
	string s1, s2;
	cout << "primer numero:" << endl;
	cin >> s1;
	cout << "segundo numero:" << endl;
	cin >> s2;
	//cout << numberOne+numberTwo << endl; 
	
	//string s1 = "4545454545"; 
	//string s2 = "6455771"; 
	int diff=0;
	int i;
	string ceros = "";
	if(s1.length() > s2.length()) {
		diff= s1.length() - s2.length();	
		for (i=0; i<diff; i++) {
			ceros = ceros + "0";
		}	
		s2 = ceros + s2;
		diff=s1.length();
	}
	else {
		diff= s2.length() - s1.length();	
		for (i=0; i<diff; i++) {
			ceros = ceros + "0";
		}
		s1=ceros + s1;
		diff=s2.length();
	}
	
	/*cout << s1;
	cout << endl;
	cout << s2;
	cout << endl;*/
	
	int llevo=0;
	stringstream sr;
	string st;
	string suma="";
	for(i = 0; i < diff; i++) {
		char c1 = s1[diff-i-1];
		char c2 = s2[diff-i-1];
		int s = ((int)c1 - 48) + ((int)c2 - 48) + llevo;
		if(s > 9) {
			llevo = 1;
			stringstream sr;
			sr << s;
			st=sr.str();
			suma = st[1] + suma;
		}
		else {
			llevo = 0;
			stringstream sr;
			sr << s;
			st=sr.str();
			suma = st[0] + suma;
		}
	}
	if(llevo == 1) {
		suma = "1" + suma;
	}
	cout << "RESULTADO:\n";
	cout << suma;
	cout << endl;
	return 0;
}