Go: package main

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

var sc = bufio.NewScanner(os.Stdin)

func main() {
	sc.Scan()
	dataset, _ := strconv.Atoi(sc.Text())
	var firstdata *big.Int

	for i := 0; i < dataset*2; i++ {
		n := new(big.Int)
		sc.Scan()
		data, _ := n.SetString(sc.Text(), 10)

		if i%2 == 0 {
			firstdata = data
			continue
		}

		result := new(big.Int)
		if result := result.Add(data, firstdata); len(result.String()) > 80 {
			fmt.Println("overflow")
			continue
		} else {
			fmt.Println(result)
		}
	}
}

C++: #include <array>
#include <algorithm>
#include <cstdio>
#include <iostream>
#include <string>

using namespace std;

typedef array<int, 11> BigInt;

string BigInt2String(BigInt bi) {
  reverse(bi.begin(), bi.end());
  char buf[89] = {};
  char *pos = buf;
  for (auto i : bi) {
    sprintf(pos, "%08d", i);
    pos += 8;
  }
  string result(buf);
  return result.substr(result.find_first_not_of('0'));
}

BigInt String2BigInt(const string& s) {
  BigInt bi { { 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0 } };

  string t(s);
  int i = 0;
  while (!t.empty()) {
    size_t offset = t.size() < 8 ? 0 : t.size() - 8;
    bi[i++] = atoi(t.substr(offset).c_str());
    t = t.substr(0, offset);
  }
  return bi;
}

string AddBig(const string& x, const string& y) {
  if (x.size() > 80 || y.size() > 80) { return string("overflow"); }

  BigInt x_bi = String2BigInt(x);
  BigInt y_bi = String2BigInt(y);
  BigInt sum { { 0, 0, 0, 0, 0, 0, 0, 0, 0, 0, 0 } };
  for (int i = 0; i < x_bi.size(); ++i) {
    sum[i] += x_bi[i] + y_bi[i];
    if (sum[i] > 99999999) {
      sum[i + 1] = 1;
      sum[i] -= 100000000;
    }
  }
  if (sum[10] != 0) { return string("overflow"); }
  return BigInt2String(sum);
}

int main() {
  int n;
  cin >> n;
  for (int i = 0; i < n; ++i) {
    string x, y;
    cin >> x;
    cin >> y;
    cout << AddBig(x, y) << endl;
  }
  return 0;
}

C++: #include<iostream>
#include<string>
//ðÇÁ·é
void addplace(unsigned int *a, unsigned int *last, int num){
	if( a == last ) return; //[·¬ÜÛÀ
	
	int carry = (*a * 10 + num) / 100000000;
	addplace(a+1, last, carry);
	*a = (*a * 10 + num) % 100000000;
}
bool trace(unsigned int *a, unsigned int *last){
	if( a == last ) return false; //[·¬ÜÛÀ
	if( !trace(a+1, last) ){
		if( *a > 0 ){
			std::cout << *a;
			return true;
		}
	}else{
		char b[11]={};
		sprintf(b,"%d",*a+100000000);
		std::cout<<b+1;
		return true;
	}
	return false;
}
void add(unsigned int *a, unsigned int *alast, unsigned int *b, int num=0){
	if( a == alast ) return; //[·¬ÜÛÀ
	
	int carry = (*a + *b + num) / 100000000;
	add(a+1, alast, b+1, carry);
	*a = (*a + *b + num) % 100000000;
}
int main(){
	int n;
	std::cin>>n;
	std::string a,b;
	std::getline(std::cin, a);//üsÇÝÌÄ
	for(;n--;){
		std::getline(std::cin, a);
		std::getline(std::cin, b);
		//uint ½è1 0000 0000-1ÜÅJEg·é
		unsigned int _a[11]={},_b[10]={};
		for(std::string::iterator it=a.begin();
			it!=a.end(); it++){
			addplace(_a, _a+10, *it-'0');
		}
		for(std::string::iterator it=b.begin();
			it!=b.end(); it++){
			addplace(_b, _b+10, *it-'0');
		}
		add(_a, _a+11, _b);
		if( _a[10] >= 1 ){
			std::cout<<"overflow\n";
		}else{
			trace(_a, _a+10);
			std::cout<<'\n';
		}
	}
}

C++: #include<iostream>
#include<cstring>
#include<cstdlib>
using namespace std;

int main()
{
    int N,j;
    cin >> N;
    for(int i=0 ; i<N ; i++){
        int ans[81]={0};
        ans[0]=11111;
        char strA[100],strB[100];
        cin >> strA >> strB;
        if(strlen(strA)>80||strlen(strB)>80){
            cout << "overflow" << endl;
            continue;
        }
        char str1[strlen(strA)][2],str2[strlen(strB)][2];
        for(int i=0 ; i<strlen(strA) ; i++){
            str1[i][0]=strA[i];
            str1[i][1]='\0';
        }
        for(int i=0 ; i<strlen(strB) ; i++){
            str2[i][0]=strB[i];
            str2[i][1]='\0';
        }
        int a[81]={0},b[81]={0};
        j=strlen(strA)-1;
        for(int i=80 ; j>=0 ; i--){
            a[i]=atoi(str1[j]);
            j--;
        }
        j=strlen(strB)-1;
        for(int i=80 ; j>=0 ; i--){
            b[i]=atoi(str2[j]);
            j--;
        }
        for(int i=80 ; i!=0 ; i--){
            ans[i]=ans[i]+a[i]+b[i];
            if(ans[i]>=10){
                ans[i]=ans[i]-10;
                ans[i-1]++;
            }
        }
        if(ans[0]!=11111){
            cout << "overflow" << endl;
            continue;
        }
        for(int i=1 ; i<81 ; i++){
            if(ans[i]!=0){
                j=i;
                break;
            }
        }
        char c[2];
        for(int i=j ; i<81 ; i++){
            itoa(ans[i],c,10);
            cout << c;
        }
        cout << '\n';
    }
    return 0;
}

C++: #include<iostream>
#include<stdio.h>
#include <algorithm>
#include<string>
#include <utility>
using namespace std;
using namespace std;
int main(){
	int m;
	string in,in2;
	int a[90],b[90],ans[90];
	int f;
	int a_len,b_len,m_len;
	cin>>m;
	while(m--){
		for(int i=0;i<90;i++){
			a[i]=b[i]=ans[i]=0;
		}
		cin>>in;
		cin>>in2;
		a_len=in.size();
		b_len=in2.size();
		for(int i=0;i<a_len;i++){
			a[a_len-1-i]=in[i]-'0';
		}
		for(int i=0;i<b_len;i++){
			b[b_len-1-i]=in2[i]-'0';
		}
		m_len=max(a_len,b_len);
		if(m_len>80){
			cout<<"overflow"<<endl;
			continue;
		}
		f=0;
		for(int i=0;i<m_len+1;i++){
			ans[i]=(a[i]+b[i]+f)%10;
			f=(a[i]+b[i]+f)/10;
		}
		if(ans[m_len]!=0){
			printf("%d",ans[0])
		}
		
		
	}
	
	
	return 0;
}

C++: #include <iostream>
#include <stdio.h>
#include <string>
#include <math.h>
#include <algorithm>
using namespace std;
int main(){
	int N;
	string x, y, temp;
	int i, a[80];
	

	cin >> N;

	for(i = 0;i < 80;i++){
		a[i] = 0;
	}

	while(N != 0){
		bool over = false;
		if(N == 0) break;
		cin >> x >> y;
		N--;
		if(x.size() > 80 || y.size() > 80){
			cout << "overflow" << endl;
			continue;
		}

		if(x.size() < y.size()){
			temp = x;
			x = y;
			y = temp;
		}
		int xi = 0, yi = 0, sum = 0;
		for(i = 0;i < x.size();i++){
			xi = x[x.size() - 1 - i] - '0';
			//cout << "xi = " << xi << endl;
			if(i >= y.size()){
				yi = 0;
			}
			else {
				yi = y[y.size() - 1 - i] - '0';
				//cout << "yi = " << yi << endl;
			}
			a[i] += xi + yi;
			//cout << "xi + yi  = " << a[x.size() - 1 - i] << endl;
			if(a[i] > 9){
				if(i < 79){
					a[i] = a[i] - 10;
					a[i + 1] += 1;
				}
				else if(i >= 79){
					cout << "overflow"	<< endl;
					over = true;
					break;				
				}
			}

			
		}
		 
		if(!over){
			bool start = false;
			for(i = 79;i >= 0;i--){
				if(!start && a[i] > 0){
					start = true;
				}
				if(start){
					cout << a[i];
				}
			}
			cout << endl;
		}
		
		for(i = 0;i < 80;i++){
				a[i] = 0;
		}
		
	}
	return 0;
}

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

using namespace std;

int main(){
    int n; cin >> n;
    while(n--){
        string a,b;
        cin >> a >> b;
        reverse(a.begin(), a.end());
        reverse(b.begin(), b.end());
        if(b.size() < a.size()) swap(a,b);
        b += "0";
        for(int i=0; i<b.size()-1; i++){
            int aa = i < a.size() ? a[i] - '0' : 0;
            int bb = b[i] - '0';
            int cc = aa + bb;
            b[i] = (cc%10) + '0';
            //cout << aa << "+" << bb << "=" << cc << endl;
            if(cc >= 10){
                b[i+1]++;
            }
        }
        if(b[b.size()-1] == '0')
            b = string(b.begin(), b.end()-1);
        reverse(b.begin(), b.end());
        if(b.size() >= 80)
            cout << "overflow" << endl;
        else cout << b << endl;
    }
    return 0;
}