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 <stdio.h>
#include <iostream>
#include <cstring>
#include <cstdlib>
#include <stack>
using namespace std;

int add(char *retstr, char *str1, char *str2){

	int str1len=strlen(str1);
	int str2len=strlen(str2);
	char ad,i1,i2;
	char offset1,offset2;
	char c1,c2;
	bool carry=false;

	if(str1len>str2len){
		offset1=0;
		offset2=str1len-str2len;
	}else{
		offset1=str2len-str1len;
		offset2=0;
	}

	for(int i=max(str1len,str2len);i<82;++i){
		retstr[i]=0;
	}

	for(int i=max(str1len,str2len)-1;i>=0;--i){

		if(i-offset1>=0){c1=str1[i-offset1];}else{c1=48;}
		if(i-offset2>=0){c2=str2[i-offset2];}else{c2=48;}
		i1=c1-48;i2=c2-48;//Ascii code to Number

		ad=i1+i2;
		if(carry)++ad;//桁上がり分

		if(ad>=10){ad %= 10; carry=true;}else{carry=false;}
		retstr[i+1]=ad+48;
	}

	if(carry){
		retstr[0]=1+48;
	}else{
		retstr[0]=0+48;
		for(unsigned int i=0;i<strlen(retstr)-1;++i){
			retstr[i]=retstr[i+1];
		}
		retstr[strlen(retstr)-1]=0;
	}

	if(strlen(retstr)>=81)return 1;

	return 0;

}

int main(){

	int n,overflowed;
	char str1[82], str2[82], retstr[82];

	cin>>n;

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

		cin>>str1;
		cin>>str2;

		if(strlen(str1)>=81||strlen(str2)>=81){
			cout<<"overflow"<<endl;
		}else{
			overflowed=add(retstr, str1, str2);
			if(overflowed){
				cout<<"overflow"<<endl;
			}else{
				cout<<retstr<<endl;
			}
		}
	}

	return 0;
}

C++: #include <iterator>
#include <typeinfo>
#include <list>
#include <map>     
#include <numeric>
#include <utility>
#include <queue>
#include <set>
#include <sstream>
#include <iomanip>
#include <iostream>
#include <stack>
#include <string>
#include <vector>
using namespace std;
#define ll long long
#define ld long double
#define rep(a,t) for(int a=0;a<t;++a) 
#define forever while(true)
#define Sort(a) sort(a.begin(),a.end())
#define Reverse(a) reverse(a.begin(),a.end())
#define pb push_back
#define int_maxvalue numeric_limits<int>::max()
ll mod = 1e9 + 7;
string Addition(string a,string b) {
	string ans = "";
	int bef = 0;
	int be1 = 0;
	for (int i = a.length() - 1; i >= 0; i--) {
		int added = stoi(a.substr(i, 1)) + stoi(b.substr(i, 1))+be1;

		if (added >= 10) {
			bef = 1;
			added -= 10;
		}
		else {
			bef = 0;
		}
		stringstream ss;
		ss << added;

		ans = ss.str()+ans;
		if (i == 0) 
		{
			break;
		}
		be1 = bef;
	}
	if (be1 == 1) {
		ans = "1" + ans;
	}
	return ans;
}
int main() {
	int n;
	cin >> n;
	rep(o1, n) {
		string a, b;
		cin >> a >> b;
		if (a.length() < b.length()) {
			swap(a, b);
		}
		string b1 = b;
		for (int i = 0; i < a.length() - b1.length(); ++i) {
			b = '0' + b;
		}
		string ans = Addition(a, b);
		if (ans.length() > 80) { cout << "overflow" << endl; }
		else {
			cout << ans << endl;
		}
	}
	return 0;
}

C++: #include <algorithm>
#include <iostream>
#include <string>
using namespace std;
int main() {
  int n, i;
  string a, b;
  bool f;
  cin >> n;
  while (n--) {
    cin >> a >> b;
    reverse(a.begin(), a.end());
    reverse(b.begin(), b.end());
    if (b.size() > a.size()) a.swap(b);
    for (i = f = 0; i < a.size(); i++) {
      int n = (a[i] - '0') + f;
      if (i < b.size()) n += b[i] - '0';
      a[i] = (n % 10) + '0';
      f = n > 9;
    }
    if (f) a += "1";
    reverse(a.begin(), a.end());
    if (a.size() > 80) {
      cout << "overflow" << endl;
    } else {
      cout << a << endl;
    }
  }
  return 0;
}

C++: #include<stdio.h>
#include<iostream>
#include<list>
#include<map>
#include<vector>
#include<algorithm>
#include<string.h>
#define range(i,a,b) for(int i = (a); i < (b); i++)
#define rep(i,b) range(i,0,b)
#define debug(x) cout << x << endl;
using namespace std;

int main(){
    int n;
    cin >> n;
    rep(k,n){
        char input1[81], input2[81];
        cin >> input1 >> input2;
        //後ろから和を求める
        int ans[81] = {0};
        int size1 = strlen(input1) - 1;
        int size2 = strlen(input2) - 1;
        int i = 0;
        while(1){
            if(size1 - i >= 0 && size2 - i >= 0)
                ans[i] += input1[size1 - i] - '0' + input2[size2 - i] - '0';
            else if(size1 - i >= 0)
                ans[i] += input1[size1 - i] - '0';
            else if(size2 - i >= 0)
                ans[i] += input2[size2 - i] - '0';
            //桁数を繰り上げる
            if(ans[i] >= 10){
                ans[i] -= 10;
                ans[i + 1]++;
            }
            if(size1 - i < 0 && size2 - i < 0)
                break;
            i++;
        }
        //出力
        if(ans[80] == 1)
            cout << "overflow" << endl;
        else{
            for(int j = i; j >= 0; j--){
                if(ans[i] == 0 && j == i)
                    continue;
                cout << ans[j];
            }
        }
        cout << endl;
    }
}

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

int main(){
  int j;
  char s1[111] = {0};
  char tmp[111] = {0};
  char s2[111] = {0};
  char ans[111];
  int  kuri = 0 ;
  memset(ans,0,sizeof(ans));
  cin >> s1 >> s2;
  if(strlen(s1) < strlen(s2)){
      for(int i=0,j=strlen(s1)-1;i<=j;i++,j--) swap(s1[i],s1[j]);
      for(int i=0,j=strlen(s2)-1;i<=j;i++,j--) swap(s2[i],s2[j]);
      for(int i=0;i<strlen(s2);i++){
	if((s2[i] + (s1[i]%'0') + kuri) > '9'){
	  ans[i] = (s2[i] + (s1[i]%'0') + kuri)-10;
	  kuri = 1;
	}
	else {
	  ans[i] = (s2[i] + (s1[i]%'0') + kuri);
	  kuri = 0;
	}
	if(i == strlen(s2)-1 && kuri == 1) ans[i+1] = '1';
      }
      if(strlen(ans) > 80) cout << "overflow" << endl;
      else{
	for(int i=strlen(ans)-1;i>=0;i--) cout << ans[i];
      }
  }
  else{
    for(int i=0,j=strlen(s1)-1;i<=j;i++,j--) swap(s1[i],s1[j]);
    for(int i=0,j=strlen(s2)-1;i<=j;i++,j--) swap(s2[i],s2[j]);
    for(int i=0;i<strlen(s1);i++){
      if((s1[i] + (s2[i]%'0') + kuri) > '9'){
	ans[i] = (s1[i] + (s2[i]%'0') + kuri)-10;
	kuri = 1;
      }
      else {
	ans[i] = (s1[i] + (s2[i]%'0') + kuri);
	kuri = 0;
      }
      if(i == strlen(s1)-1 && kuri == 1) ans[i+1] = '1';
    }
    if(strlen(ans) > 80) cout << "overflow" << endl;
    else{
      for(int i=strlen(ans)-1;i>=0;i--) cout << ans[i];
    }
  }
  cout << endl;
}

C++: #include<cstdio>
#include<algorithm>
#include<string.h>
using namespace std;

int n;
char a[85],b[85],answer[86];
char aa[85],bb[85];
int na=0,nb=0;

int main()
{
	scanf("%d",&n);
	for(int i=0;i<n;i++)
	{
		na=0,nb=0;
		//配列aa(bb)を'0'で初期化
		fill(aa,aa+85,'0');
		fill(bb,bb+85,'0');
		fill(a,a+85,0);
		fill(b,b+85,0);
		fill(answer,answer+85,0);
		scanf("%s%s",a,b);
		for(int i=0;a[i]!='\0';i++)
		{
			na++;
		}
		for(int i=0;b[i]!='\0';i++)
		{
			nb++;
		}
		//配列a(b)を右詰めでaa(bb)に収納する
		memcpy(aa+85-na,a,sizeof(char)*na);
		memcpy(bb+85-nb,b,sizeof(char)*nb);
		//以下足し算
		for(int j=84;j>=0;j--)
		{
			answer[j]+=aa[j]+bb[j]-'0';
			//繰り上げ
			if(answer[j]>'9')
			{
				answer[j-1]++;
				answer[j]-=10;
			}
		}
		int count=0;
		for(int j=0;(j<84)&&answer[j]=='0';j++)
		{
			count++;
		}
		if(count<5)
		{
			printf("overflow\n");
		}
		else
		{
		answer[85]='\0';//文字列終了のお知らせ
		memcpy(answer,answer+count,85-count+1);
		//出力
		printf("%s\n",answer);
		}
	}
}