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 "bits/stdc++.h"
#include<unordered_map>
#include<unordered_set>
#pragma warning(disable:4996)
using namespace std;
using ld = long double;
const ld eps = 1e-9;

//// < "d:\d_download\visual studio 2015\projects\programing_contest_c++\debug\a.txt" > "d:\d_download\visual studio 2015\projects\programing_contest_c++\debug\b.txt"


int main() {
	int N; cin >> N;
	while (N--) {

		string a, b; cin >> a >> b;
		vector<int>v(101);
		for (int i = 0; i < a.size(); ++i) {
			v[101 - a.size() + i] = a[i] - '0';
		}
		for (int i = 0; i < b.size(); ++i) {
			v[101 - b.size() + i] += b[i] - '0';
		}
		for (int i = 100; i >= 0; --i) {
			if (v[i] >= 10) {
				v[i - 1]++;
				v[i] -= 10;
			}
		}
		int num = 0;
		num = find_if(v.begin(), v.end(), [](const int a) {return a; })-v.begin();
		if (num <= 20)cout << "overflow" << endl;
		else {
			for (int i = num; i < 101; ++i) {
				cout << v[i];
			}
			cout << endl;
		}
	}
	return 0;
}

C++: #include<stdio.h>
#include<math.h>
#include<string.h>

int main(void){

	int N,math1,math2,kuri,keta;
	int i,j,k;
	char fi[10000],se[10000],ans[10000];

	scanf("%d",&N);

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

		scanf("%s",&fi);
		scanf("%s",&se);

		if(strlen(fi)<=80&&strlen(se)<=80){
		for(j=strlen(fi)-1,k=strlen(se)-1,keta=0,kuri=0;j>=0||k>=0;j--,k--,keta++){
			if(j<0) math1=0;
			else math1=fi[j]-'0';
			if(k<0) math2=0;
			else math2=se[k]-'0';
            ans[keta]=((math1+math2+kuri)%10)+'0';
			kuri=(math1+math2+kuri)/10;
		}
		if(kuri==1) printf("1");
		if(keta-1<=80){
			for(j=keta-1;j>=0;j--){
				printf("%c",ans[j]);
			}
		}
		else printf("overflow");
		}
		else printf("overflow");
		printf("\n");
	}

	return 0;
}

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

int main(){
	string a, b;
	vector<int> ans;
	int n, num, ai, bi, c=0;
	bool flag;

	cin >> n;
	
	for(int i=0 ; i<n ; i++){
		cin >> a >> b;
		flag = false;
		
		for(int j=0 ; j<100 ; j++){
			num = 0;
			ai = a.size()-j-1;
			bi = b.size()-j-1;
			if( ai<0 && bi<0 && c==0) break;
			
			if(ai>=0)num += a[ ai ] - 48;
			if(bi>=0)num += b[ bi ] - 48;
			ans.push_back( (num+c)%10 );
			if(num+c >= 10) c = 1;
			else c = 0;
		}
		if(ans.size()>80){
			flag = true;
		}
		if(flag){
			printf("overflow");
		}else{
			for(int j=ans.size()-1 ; j>=0 ; --j){
				printf("%d", ans[j]);
			}
		}
		printf("\n");
		ans.clear();
	}
	return 0;
}

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

#define MAX_DIGITS 80

int main()
{
    using namespace std;
    string num1, num2, result;
    int count;
    cin >> count;
    for (int i = 0; i < count; ++i) {
        cin >> num1 >> num2;
        if (num1.size() > MAX_DIGITS || num2.size() > MAX_DIGITS) {
            result = "overflow";
        }
        else {
            int co = 0; // carryover
            reverse(num1.begin(), num1.end());
            reverse(num2.begin(), num2.end());
            size_t digits = max(num1.size(), num2.size()) + 1;
            result.resize(digits);
            for (int j = 0; j < digits; ++j) {
                string digit1, digit2;
                digit1 = j < num1.size() ? num1.substr(j, 1) : "0";
                digit2 = j < num2.size() ? num2.substr(j, 1) : "0";
                int tmp = atoi(digit1.c_str()) + atoi(digit2.c_str()) + co;
                co = tmp/10;
                result[j] = char(48 + tmp - co*10);
            }
            if (co == 1) result = "overflow";
            reverse(result.begin(), result.end());
            if (result[0] == '0') result = result.substr(1);
        }
        cout << result << endl;
    }
}

C++: #include<iostream>
#include<algorithm>
#include<cstring>
#include<string>
#include<cstdio>
#define MOJI 48
using namespace std;
int multiple(string a,string b);
char c[111111];
int main(){
  int n;
  string a,b;
  while(cin >> n){
    for(int i=0;i<n;i++){
      cin >> a;
      cin >> b;
      int k = multiple( a , b );
      if(k>=80){
	cout << "overflow" << endl;
      }else {
	for(int i=k-1;i>=0;i--){
	  printf("%d",c[i]);
	}
	cout << endl;
      }
    }
  }
}
int multiple(string a,string b){
  int a_size,b_size,w=0;
  char k = 0; // 桁上げ
  a_size = a.size();
  b_size = b.size();
  while(a_size>0 || b_size>0 || k == 1){
    if(a_size>0 && b_size>0){
      c[w] = (a[a_size-1] - MOJI + b[b_size-1] - MOJI + k) % 10;
      k = (k + (a[a_size-1]-MOJI) + (b[b_size-1]-MOJI)) / 10;
    }
    else if(a_size>0 && b_size<=0){
      c[w] = ((a[a_size-1]-MOJI) + k) % 10;
      k = (a[a_size-1]-MOJI + k) / 10;
    }
    else if(a_size<=0 && b_size>0){
      c[w] = (b[b_size-1] - MOJI + k) % 10;
      k = (k + b[b_size-1] - MOJI) / 10;
    }else {
      c[w] = k;
      k = 0;
    }
    a_size--;
    b_size--;
    w++;
  }
  return w;
}

C++: #include <bits/stdc++.h>
#define PB push_back
#define MP make_pair
#define REP(i,n) for (int i=0;i<(n);i++)
#define FOR(i,a,b) for(int i=(a);i<(b);i++)
#define ALL(a) (a).begin(),(a).end()
using namespace std;
double EPS=1e-10;
int INF=1e9;
string sum(string a,string b){
      string res="";
      reverse(ALL(a));
      reverse(ALL(b));
      if(a.size()>b.size())swap(a,b);
      int k=0;
      REP(i,a.size()){
            int t=a[i]-'0'+b[i]-'0'+k;
            res+=(char)(t%10+'0');
            k=t/10;
      }
      for(int i=a.size();i<b.size();i++){
            int t=b[i]-'0'+k;
            res+=(char)(t%10+'0');
            k=t/10;
      }
      while(k>0){
            res+=(char)(k%10+'0');
            k/=10;
      }
      reverse(ALL(res));
      return res;
}
int main(){
      int n;
      cin>>n;
      while(n--){
            string a,b;
            cin>>a>>b;
            cout<<sum(a,b)<<endl;
      }
}