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 <stdio.h>
#include <stdlib.h>
#include <string.h>

int main(void) {

  int n;

  char buf[1000];
  char outbuf[1000];

  char c;

  int a[1000],b[1000];
  int result[1000];
  
  int sum,carry;
  int idx1,idx2;
  int maxlen;

  int i,j;

  scanf("%d",&n);

  c=getchar();

  for(i=0;i<n;i++) {

    fgets(buf,sizeof(buf),stdin);

    idx1=0;

    while(buf[idx1]!='\n') {

      a[idx1]=buf[idx1]-48;

      idx1++;

    }

    //for(j=0;j<idx1;j++) printf("%d",a[j]);

    fgets(buf,sizeof(buf),stdin);

    idx2=0;

    while(buf[idx2]!='\n') {

      b[idx2]=buf[idx2]-48;

      idx2++;

    }

    if(idx1>idx2) maxlen=idx1;
    
    else maxlen=idx2;

    carry=0;

    for(j=0;j<maxlen;j++) {

      if(j<idx1 && j<idx2) sum=a[idx1-j-1]+b[idx2-j-1]+carry;

      else if(j>=idx1) sum=b[idx2-j-1]+carry;

      else sum=a[idx1-j-1]+carry;


      if(sum>=10) carry=1;

      else carry=0;

      result[j]=sum%10;

    }
    //printf("%d\n",carry);
    if(carry==1) {

      if(maxlen>=80) printf("overflow\n");

      else {

	outbuf[0]='1';

	for(j=0;j<maxlen;j++) {

	  outbuf[j+1]=result[maxlen-j-1]+48;

	}

	outbuf[maxlen+1]='\0';

	printf("%s",outbuf);

	puts("");

      }

    }

    else {

      for(j=0;j<maxlen;j++) {

	outbuf[j]=result[maxlen-j-1]+48;

      }

      outbuf[maxlen]='\0';

      printf("%s",outbuf);

      puts("");

    }

  }

  return 0;

}

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

using namespace std;

int main (void) {
    int N;
    cin >> N;
    while (N--) {
        string s1, s2;
        cin >> s1;
        cin >> s2;
        vector<int> n1, n2, res;
        for (int i = s1.size(); i--;) {
            n1.push_back((int)(s1[i] - '0'));
        }
        for (int i = s2.size(); i--;) {
            n2.push_back((int)(s2[i] - '0'));
        }
        if (n1.size() < n2.size()) {
            for (int i = n1.size(); i < n2.size(); i++) {
                n1.push_back(0);
            }
        } else if (n2.size() < n1.size()) {
            for (int i = n2.size(); i < n1.size(); i++) {
                n2.push_back(0);
            }
        }
        int s, c = 0;
        for (int i = 0; i < n1.size(); i++) {
            s = n1[i] + n2[i] + c;
            res.push_back(s % 10);
            c = s / 10;
        }
        while (c != 0) {
            res.push_back(c % 10);
        }
        int size = res.size();
        while (size > 0 && res[size - 1] == 0) {
            size--;
        }
        if (size > 80) {
            cout << "overflow" << endl;
        } else {
            int i = size;
            while (i--) {
                cout << res[i];
            }
            cout << endl;
        }
    }
    return 0;
}

C++: #include<stdio.h>
#include<iostream>
#include<string>
using namespace std;
int main(){

int n;
cin>>n;
for(int iii=0;iii<n;iii++){
string st1,st2;
int n1[83]={},n2[83]={};
cin>>st1>>st2;

int r=st1.size();
int N[83]={};int y=0;
if(r<st2.size())r=st2.size();
if(st1.size()>80||st2.size()>80)r=81;
else{
for(int i=0;i<st1.size();i++)
n1[i]=st1[st1.size()-i-1]-'0';
for(int i=0;i<st2.size();i++)
n2[i]=st2[st2.size()-i-1]-'0';



for(int i=0;i<r;i++){
N[i]=(n1[i]+n2[i]+y)%10;
y=(n1[i]+n2[i]+y)/10;
if(i==r-1&&y>0)r++;
}}
if(r>80)cout<<"overflow"<<endl;
else{
for(int i=0;i<r;i++)
cout<<N[r-i-1];
cout<<endl;
}}
return 0;
}

C++: #include "bits/stdc++.h"
using namespace std;
int main() {
	cin.tie(0); ios::sync_with_stdio(false);
	int N; cin >> N;
	for (int i = 0; i < N;i++) {
		string str1, str2;
		cin >> str1 >> str2;
		int c = 0;	//????????°?????????
		if (str1.length() > str2.length()) {
			string temp(str1.length() - str2.length() + 1,'0');
			str1 = '0' + str1;
			str2 = temp + str2;
		}
		else if (str1.length() < str2.length()) {
			string temp(str2.length() - str1.length() + 1, '0');
			str1 = temp + str1;
			str2 = '0' + str2;
		}
		string ans((int)str1.length(),'0');
		for (int i = str1.length()-1 ; i >= 0;i--) {
			int dig1 = str1[i] - '0', dig2 = str2[i] - '0';
			if (c + dig1 + dig2 >= 10) {
				ans[i] = (c + dig1 + dig2) % 10 +'0';
				c = (c + dig1 + dig2)/10;
			}
			else {
				ans[i] = (c + dig1 + dig2) + '0';
				c = (c + dig1 + dig2) / 10;
			}
		}
		int size = ans.length();
		if (ans[0] == '0') size--;
		if (size > 80)cout << "overflow" << endl;
		else {
			if (ans[0] == '0') {
				cout << ans.substr(1) << endl;
			}
			else {
				cout << ans << endl;
			}
		}
	}
}

C++: #include<iostream>
#include<cstdio>
#include<algorithm>
#include<string>
using namespace std;
string a,b;
int s,cop,copp,last,n;
int latte[81]={0},cafe[81]={0},all[81]={0};
int main(){
    cin>>n;
    for(int l=0;l<n;l++){
        fill(latte,latte+81,0);
        fill(cafe,cafe+81,0);
        fill(all,all+81,0);
        cin>>a>>b;
        if(a.size()>b.size()){
            s=a.size();
            cop=a.size(),copp=b.size(),last=s;
        }
        else {
            s=b.size();
            cop=a.size(),copp=b.size(),last=s;
        }
        for(int i=0;i<a.size();i++){
            cop--;
            if(a[i]=='0')latte[cop]=0;
            if(a[i]=='1')latte[cop]=1;
            if(a[i]=='2')latte[cop]=2;
            if(a[i]=='3')latte[cop]=3;
            if(a[i]=='4')latte[cop]=4;
            if(a[i]=='5')latte[cop]=5;
            if(a[i]=='6')latte[cop]=6;
            if(a[i]=='7')latte[cop]=7;
            if(a[i]=='8')latte[cop]=8;
            if(a[i]=='9')latte[cop]=9;
            }
        for(int i=0;i<b.size();i++){
            copp--;
            if(b[i]=='0')cafe[copp]=0;
            if(b[i]=='1')cafe[copp]=1;
            if(b[i]=='2')cafe[copp]=2;
            if(b[i]=='3')cafe[copp]=3;
            if(b[i]=='4')cafe[copp]=4;
            if(b[i]=='5')cafe[copp]=5;
            if(b[i]=='6')cafe[copp]=6;
            if(b[i]=='7')cafe[copp]=7;
            if(b[i]=='8')cafe[copp]=8;
            if(b[i]=='9')cafe[copp]=9;
            }
         for(int i=0;i<=s;i++){
            all[i]+=latte[i]+cafe[i];
            if(all[i]>9){
                all[i]-=10;
                all[i+1]+=1;
            }
         }
         for(int i=s-1;i>-1;i--){
            if(all[last]!=0)cout<<all[last];
            cout<<all[i];
            all[last]=0;
         }
         cout<<endl;
    }
return 0;
}

C++: #include <stdio.h>
#include <string.h>
#define N 80

int main(void){
    char buf[N+1];
    int datas;
    int degits;
    int lint1[N+1], lint2[N+1], sum[N+1];
    int cry;
    
    scanf("%d", &datas);
    for (int n=0; n<datas; n++){
        // reset
        for (int i=0; i<N; i++) lint1[i]=lint2[i]=0;
        // input
        scanf("%s", buf);
        degits = strlen(buf);
        for (int i=0; i<degits; i++){
            lint1[degits-i-1]= buf[i]-'0';
        }
        scanf("%s", buf);
        degits = strlen(buf);
        for (int i=0; i<degits; i++){
            lint2[degits-i-1]= buf[i]-'0';
        }
        // calc
        cry=0;
        for (int i=0; i<N; i++){
            sum[i] = lint1[i]+lint2[i] + cry;
            cry = sum[i]>9 ? 1:0;
            if (cry) sum[i] -= 10;
        }
        // output
        if (cry){
            puts("overflow");
        } else {
            bool zero=true;
            if (cry) puts("1");
            for (int i=N-1; i>=0; i--){
                if (zero && sum[i] != 0) zero=false;
                if (!zero) printf("%d", sum[i], i);
            }
            printf("\n");
        }
    }
    
    return 0;
}