Go: package main

import (
	"bufio"
	"fmt"
	"math/big"
	"os"
	"strconv"
)

var scanner = bufio.NewScanner(os.Stdin)

func nextString() string {
	scanner.Scan()
	return scanner.Text()
}

func nextInt() int {
	n, err := strconv.Atoi(nextString())
	if err != nil {
		fmt.Printf("strconv.Atoi failed: %v\n", err)
	}
	return n
}

func main() {
	scanner.Split(bufio.ScanWords)

	const maxCapacity = 512 * 1024
	buf := make([]byte, maxCapacity)
	scanner.Buffer(buf, maxCapacity)

	n := nextInt()
	maxint, _ := new(big.Int).SetString("100000000000000000000000000000000000000000000000000000000000000000000000000000000", 10)

	for i := 0; i < n; i++ {
		aStr := nextString()
		bStr := nextString()

		a, _ := new(big.Int).SetString(aStr, 10)
		b, _ := new(big.Int).SetString(bStr, 10)
		c := big.NewInt(0).Add(a, b)

		if c.Cmp(maxint) < 0 {
			fmt.Println(c)
		} else {
			fmt.Println("overflow")
		}
	}
}

C++: #include<iostream>
#include<string>
#include<cmath>
#define loop(i,a,b) for(int i=a;i<b;i++)
#define rep(i,a) loop(i,0,a)
using namespace std;

int stoi(string s){
  int size=s.size();
  int num=0;
  rep(i,size){
    num+=((int)s[i]-'0')*pow(10,size-i-1);
  }
  return num;
}

string itos(int num){//this function is used when you want to change variable type from int to string
  int size=0;
  int cnt=1;
  while(num/cnt!=0){//count the digit of num
    size++;
    cnt*=10;
  }
  string s="";
  rep(i,size){//change num from int to string
    s+=(char)((num/(int)pow(10,size-i-1))+'0');
    num-=(int)(num/pow(10,size-i-1))*pow(10,size-i-1);
  }
  return s;
}

int main(){
	int n;
	cin>>n;
	string s[2];
	rep(i,n){
		cin>>s[0]>>s[1];
		if(s[0].size()>80 || s[1].size()>80){
			cout<<"overflow"<<endl;
			continue;
		}
		int dis=abs(s[0].size()-s[1].size());
		if(s[0].size()>s[1].size()){
			rep(j,dis){
				s[1].insert(0,"0");
			}
		}else{
			rep(j,dis){
				s[0].insert(0,"0");
			}
		}
		int size=s[0].size();
		int alt=0;
		string ans="";
		int num1,num2;
		for(int a=(size-1);a>=0;a--){
			num1=(int)s[0][a]-'0';
			num2=(int)s[1][a]-'0';
			int result=num1+num2+alt;
			if(result<10){
				if(!result)ans.insert(0,"0");
				else ans.insert(0,itos(result));
				alt=0;
			}else{
				if(!(result%10))ans.insert(0,"0");
				else ans.insert(0,itos(result%10));
				alt=1;
			}
		}
		if(alt)ans.insert(0,"1");
		if(ans.size()>80)cout<<"overflow"<<endl;
		else cout<<ans<<endl;
	}
	return 0;
}

C++: #include <iostream>
#include <string>
#include <list>
using namespace std;

class bigdata
{
public:
  list < int >data;
  string to_str ()
  {
    string str (data.size (), '0');
    int i = 0;
    auto it = data.begin ();
    while (it != data.end ())
      {
	string buf = to_string (*it);
	  str[i] = buf[0];
	  it++;
	  i++;
      }
    return str;
  }
  bigdata & operator= (string str)
  {
    for (int i = 0; i < str.length (); i++)
      data.push_back (stoi (str.substr (i, 1)));
  }
  bigdata & operator+ (bigdata & obj)
  {
    auto it_a = this->data.begin ();
    auto it_b = obj.data.begin ();
    if (this->data.size () < obj.data.size ())
      {
	int buf = obj.data.size () - this->data.size ();
	for (int i = 1; i <= buf; i++)
	  this->data.push_front (0);
      }
    if (this->data.size () > obj.data.size ())
      {
	int buf = this->data.size () - obj.data.size ();
	for (int i = 1; i <= buf; i++)
	  obj.data.push_front (0);
      }
    this->data.push_front (0);
    obj.data.push_front (0);

    while (it_a != this->data.end ())
      {
	*it_a += *it_b;
	if (*it_a >= 10)
	  {
	    --it_a;
	    *it_a += 1;
	    ++it_a;
	    *it_a -= 10;
	  }
	it_a++;
	it_b++;
      }
    it_a = this->data.begin ();
    if (*it_a == 0)
      this->data.pop_front ();
    return *this;
  }
};

int
main ()
{
  bigdata data1, data2;
  string s1, s2;
  cin >> s1 >> s2;
  data1 = s1;
  data2 = s2;
  data1 = data1 + data2;
  cout << data1.to_str () << endl;
}

C++: #include <iostream>
#include <string>
#include <cstdlib>
#include <algorithm>

#define MAX_DIGIT 80
#define ERROR ("Overflow")



int main(void){
	using namespace std;

	int N;
	cin >> N;

	for(int i=1; i<=N; i++){

		string slhs, srhs;
		cin >> slhs >> srhs;
		if(slhs.size()>MAX_DIGIT || srhs.size()>MAX_DIGIT){
			cout << ERROR << endl;
			continue;
		}

		reverse(slhs.begin(), slhs.end());
		reverse(srhs.begin(), srhs.end());

		int lhs[MAX_DIGIT], rhs[MAX_DIGIT], res[MAX_DIGIT];
		fill(lhs, lhs+MAX_DIGIT, 0);
		fill(rhs, rhs+MAX_DIGIT, 0);
		fill(res, res+MAX_DIGIT, 0);
		auto atoi_digit = [] (char x) -> int {
			return atoi(&x);
		};
		transform(slhs.begin(), slhs.end(), lhs, atoi_digit);
		transform(srhs.begin(), srhs.end(), rhs, atoi_digit);

		int headDigit;
		for (headDigit=MAX_DIGIT-1; headDigit>=0; headDigit--){
			if(lhs[headDigit]!=0 || rhs[headDigit]!=0){
				break;
			}
		}
		if(headDigit != MAX_DIGIT-1){
			headDigit++;
		}

		int carry = 0;
		for (int j=0; j<=headDigit; j++){
			auto d = div(lhs[j]+rhs[j]+carry,10);
			carry = d.quot;
			res[j] = d.rem;
		}
		if(headDigit==MAX_DIGIT-1 && carry !=0){
			cout << ERROR << endl;
			continue;
		}


		for (; headDigit>=1; headDigit--){
			if(res[headDigit]!=0){
				break;
			}
		}
		for (int j=headDigit; j>=0; j--){
			cout << res[j];
		}
		cout << endl;
	}
}

C++: #include <cmath>
#include <iostream>
#include <string>
#include <cstdlib>

using namespace std;

int main(){
    string inp[2];
    int output[81],n;
    cin >> n;
    while(n--){
        cin >> inp[0] >> inp[1];
        if(inp[0].size()>80||inp[0].size()>80) cout << "overflow" << endl;
        else{
            fill(output,output+81,0);
            reverse(inp[0].begin(),inp[0].end());
            reverse(inp[1].begin(),inp[1].end());
            for(int i=0;i<81;i++){
                if(i<inp[0].size()) output[i]+=inp[0][i]-'0';
                if(i<inp[1].size()) output[i]+=inp[1][i]-'0';
                if(i!=80&&output[i]>9){
                    output[i+1]+=output[i]/10;
                    output[i]%=10;
                }
            }
            if(output[80]>0) cout << "overflow" << endl;
            else{
                bool flag=false;
                for(int i=79;i>=0;i--){
                    if((!flag&&output[i]!=0)||i==0) flag=true;
                    if(flag) cout << output[i];
                }
                cout << endl;
            }
        }
    }
    return 0;
}

C++: #include<cstdio>
#include<algorithm>
#include<cstring>
#include<string>
using namespace std;
int main(void)
{
	int y;
	scanf("%d",&y);
	for(int z = 0;z < y;z++)
	{
		char suji1[100000],suji2[100000];
		char ans[100000] = {0};

		for(int i = 0;i<82;i++)
		{
			suji1[i] = '0';
			suji2[i] ='0';
		}

		int overflow = 0;

		scanf("%s",suji1);
		scanf("%s",suji2);
		int suji1s = strlen(suji1);
		int suji2s = strlen(suji2);

		int anss;
		if(suji1s>suji2s)  anss = suji1s;
		else anss = suji2s;

		suji1[suji1s] = '0';
		suji2[suji2s] = '0';

		reverse(suji1,suji1+suji1s);
		reverse(suji2,suji2+suji2s);

		for(int i = 0;i < 81;i++)
		{
			ans[i] = suji1[i]+suji2[i] - '0';
			if(ans[i] >= ':')
			{
				ans[i]  = ans[i] - 10;
				suji2[i+1] = suji2[i+1]+1;
			}
		}
		if(ans[anss]> '0')
		{
			anss++;
		}
		
		if(anss > 80) overflow = 1;
		reverse(ans,ans+anss);
		ans[anss] = 0;
		if(overflow >= 1)
		{
			printf("overflow\n");
		}
		else
		{
			printf("%s",ans);
		}
		printf("\n");
		
	}
	return 0;
}

C++: #include<bits/stdc++.h>
using namespace std;


string solve(string s1, string s2){

while(s1.size()<85){
s1 = '0' + s1;
}
while(s2.size()<85){
s2 = '0' + s2;
}
string ret = "";
int kuri = 0;
for(int i=0;i<80;i++){
int r = (int)(s1[s1.size()-1-i]-'0' + s2[s2.size()-1-i]-'0') + kuri;
kuri = r/10;
r = r%10;
ret = ((char)('0'+r)) + ret;
}
if (kuri>0) return "overflow"; else return ret;
}

int main(void){
int N;
cin >> N;
for(int i=0;i<N;i++){
string s1, s2;
cin >> s1 >> s2;
cout << solve(s1, s2) << endl;
}
return 0;
}