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<sstream>
#include<iomanip>
using namespace std;
class BigInt{
  unsigned int i[11];
public:
  BigInt(string str){
    i[0]=0;
    for(int j=80;j>0;j-=8){
      if((int)str.size()>j){
	stringstream ss;
	ss << str.substr(str.size()-j,8);
	ss >> i[11-j/8];
      }else if((int)str.size()+8>j){
	stringstream ss;
	ss << str.substr(0,str.size()+8-j);
	ss >> i[11-j/8];
      }else{
	i[11-j/8]=0;
      }
    }
  }
  BigInt():i{0,0,0,0,0,0,0,0,0,0,0}
  {}
  void print(){
    if(i[0])
      cout << "overflow\n";
    else{
      int j=1;
      for(;j<11;j++)
	if(i[j]>0)
	  break;
      cout << i[j++];
      for(;j<11;j++)
	cout << setfill('0') << setw(8) << right << i[j];
      cout << "\n";
    }
  }
  BigInt operator+(const BigInt& rhs){
    BigInt result = rhs;
    for(int j=10;j>0;j--){
      result.i[j] += i[j];
      if(result.i[j]>=100000000){
	result.i[j]-=100000000;
	result.i[j-1]++;
      }
    }
    return result;
  }
};

int main(){
  int n;
  cin >> n;
  for(int i=0;i<n;i++){
    string tmp;
    cin>>tmp;
    BigInt b(tmp);
    cin>>tmp;
    BigInt a(tmp);
    (a+b).print();
  }
}

C++: #include<iostream>
#include<string>
#include<algorithm>
using namespace std;
int main(){
  int n;
  cin >>n;
  cin.ignore();
  while(n--){
    string one;
    string two;
    string ans[100];
    getline(cin,one);
    getline(cin,two);
    int os = one.size()-1, ts = two.size()-1;
    int ls = max(os,ts), ss = min(os,ts);
    int as = 0,x,t = 0;
    for(int i=0; i<=ls; i++){
      if(i<=os && i<=ts){x = one[os-i]+two[ts-i]-'0'*2+t;}
      else if(i<=os){x = one[os-i]-'0'+t;}
      else if(i<=ts){x = two[ts-i]-'0'+t;}
      t = x/10;
      x = x%10;
      ans[i] = x+'0';
      as++;
    }
    if(t>0){ans[as+1] = t+'0';as++;}
    if(as>=80){cout <<"overflow"<<endl;}
    else{
      for(int i=as; i>=0; i--){
	cout <<ans[i];
      }
      cout <<endl;
    }
  }
  return 0;
}

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

using namespace std;

void sum80(int* a, int* b);
int get80(string str, int* num);

int main(void){
	string a80, b80;
	int a[81], b[81], s[81];
	char c;
	int i, N;
	cin>>N;
	i=0;
	while(i<N){
		cin>>a80;
		cin>>b80;
		if(get80(a80, a)<0 || get80(b80, b)<0){
			cout<<"overflow"<<endl;
		}else{
			sum80(a, b);
		}
		i++;
	}
	return 0;
}

int get80(string str, int* num){
	char c;
	int k;
	int d=str.length();
	if(d<=80){
		for(int i=0; i<d; i++){
			num[i]=str[d-1-i]-'0';
		}
		for(int i=d;i<81; i++){
			num[i]=0;
		}
		return d;
	}else{
		return -1;
	}

}

void sum80(int* a, int* b){
	int s[81], amari;
	amari=0;
	for(int i=0; i<81; i++){
		if(a[i]+b[i]>=10){
			amari=1;
		}else{
			amari=0;
		}
		s[i]=(a[i]+b[i]+amari)%10;
	}
	if(s[80]!=0){
		cout<<"overflow"<<endl;
	}else{
		bool flag=false;
		for(int i=79; i>=0; i--){
			if(flag || s[i]!=0){
				flag=true;
				cout<<s[i];
			}
		}
		cout<<endl;
	}
}

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


int main(){
  int n,i,j;
  string a,b;
  int sum[110]={};
  int up = 0;
  cin >> n;
  for(i=0;i<n;i++){
    cin >> a >> b;
    int sa=a.size(),sb=b.size();
    int flag=0;
    if(sa>=80||sb>=80) flag =1;
    for(j=0;j<max(sa,sb);j++){
      if(sa>j&&sb>j){
	sum[j] = (a[sa-j-1]-'0') + (b[sb-j-1]-'0') + up;
      }else if(sa>j){
	sum[j] = (a[sa-j-1]-'0') + up;
      }else if(sb>j){
	sum[j] = (b[sb-j-1]-'0') + up;
      }
      up=0;
      if(sum[j]>9){
	up=1;
	sum[j]-=10;
      }
    }
    sum[max(sa,sb)]=up;
    int k=110-1;
    while(sum[k]==0&&k>0) k--;
    if(k>=80||flag==1) cout << "overflow";
    else
    for(j=k;j>=0;j--)
      cout << sum[j];
    cout << '\0' << endl;
  }
}

C++: #include <iostream>
#include <string.h>

using namespace std;

int main(int argc, char* argv[])
{
	int i;
	int num, pos;
	int len1, len2;
	char temp;
	char f1[101] = { 0 }, f2[101] = { 0 };
	int  up[100] = { 0 };
	int  sum[101] = { 0 };

	cin >> num;
	while (num--) {
		cin >> f1;
		cin >> f2;

		len1 = strlen(f1);
		len2 = strlen(f2);
		if (len1 > 80 || len2 > 80) {
			cout << "overflow" << endl;
			continue;
		}

		for (int i = 0; i < len1 / 2; i++) {
			temp = f1[i];
			f1[i] = f1[len1 - 1 - i];
			f1[len1 - 1 - i] = temp;
		}
		for (int i = 0; i < len2 / 2; i++) {
			temp = f2[i];
			f2[i] = f2[len2 - 1 - i];
			f2[len2 - 1 - i] = temp;
		}

		for (i = 0; i < len1; i++) f1[i] = f1[i] - 0x30;
		for (i = 0; i < len2; i++) f2[i] = f2[i] - 0x30;

		for (int i = 0; i < 100; i++) {
			sum[i] += (f1[i] + f2[i]);
			if (sum[i] >= 10) {
				sum[i] -= 10;
				sum[i + 1]++;
			}
		}

		for (pos = 100; sum[pos] == 0; pos--) {}
		if (pos >= 80) {
			cout << "overflow";
			break;
		}
		for (int i = pos; i >= 0; i--) {
			cout << sum[i];
		}
		cout << endl;
	}
return 0;
}

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

using namespace std;

int main(){
	int n;
	string ret1,ret2;
	vector<int> result;
	int num = 0, ret = 0;
	int j;

	cin >> n;

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

		cin >> ret1 >> ret2;

		if (ret1.size() > 80 || ret2.size() > 80){
			cout << "overflow" << endl;
		}

		for (j = 1;j <= ret1.size() && j <= ret2.size();j++){
			num = (ret1.at(ret1.size() - j) - '0' + ret2.at(ret2.size() - j) - '0' + ret) % 10;
			ret = (ret1.at(ret1.size() - j) - '0' + ret2.at(ret2.size() - j) - '0' + ret);
			ret = (int)log10((double)ret);
			result.push_back(num);
		}

		if (ret1.size() == ret2.size()){
			if (ret != 0)
				result.push_back(ret);
		}
		else if(ret1.size() > ret2.size()){
			for (; j <= ret1.size(); j++){
				num = (ret1.at(ret1.size() - j) - '0' + ret) % 10;
				ret = ret1.at(ret1.size() - j) - '0' + ret;
				ret = (int)log10((double)ret);
				result.push_back(num);
			}
			if (ret != 0)
				result.push_back(ret);
		}
		else if (ret1.size() < ret2.size()){
			for (; j <= ret2.size(); j++){
				num = (ret2.at(ret2.size() - j) - '0' + ret) % 10;
				ret = ret2.at(ret2.size() - j) - '0' + ret;
				ret = (int)log10((double)ret);
				result.push_back(num);
			}
			if (ret != 0)
				result.push_back(ret);
		}


		while (result.empty() != true){
			cout << result[result.size()-1];
			result.pop_back();
		}
                cout << endl;

	}


	return 0;
}