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<algorithm>
using namespace std;
int main(){
  int n;
  cin >>n;
  while(n--){
    string a,b;
    cin >>a>>b;
    int num[900] = {0},tab = 0,i = 0;
    int as = a.size()-1;
    int bs = b.size()-1;
    int nmi = min(as,bs);
    int nma = max(as,bs);
    for(int x; i<=nma; i++){
      if(i<=nmi){x = tab+a[as-i]+b[bs-i]-'0'*2;}
      else if(as<bs){x = tab+b[bs-i]-'0';}
      else if(bs<as){x = tab+a[as-i]-'0';}
      tab = x/10;
      num[i] = x%10;
    }
    if(tab){num[nma] = tab;i++;}
    if(i>80){cout <<"overflow";}
    else{for(int x=i-1; x>=0; x--){cout <<num[x];}}
    cout <<endl;
  }
  return 0;
}

C++: #include <stdio.h>
#include <string.h>
int main(void){
    int n,ap,bp,cp,t;
    char a[81],b[81],c[81];
    scanf("%d",&n);
    while(~scanf("%s%s",&a,&b)){
        ap=strlen(a);
        bp=strlen(b);
        cp=sizeof(c);
        c[cp]='\0';
        t=0;
        while(cp){
            t+=(ap?a[--ap]-'0':0)+(bp?b[--bp]-'0':0);
            c[--cp]='0'+(t%10);
            t/=10;
        }
        while(t) c[--cp]='0'+(t%10);
        printf("%s\n",&c[cp]);
    }
    return 0;
}

C++: #include "string.h"
#include "math.h"
#include "stdio.h"
#include <iostream>
using namespace std;

/** Problem0015 : National Budget **/
int main()
{
	string a, b, s;
	int n, tmp, la, lb;
	bool up=false;
	
	cin >> n;
	for (int i=0; i<n; i++) {
		cin >> a >> b;
		s="";
		
		reverse(a.begin(), a.end());
		reverse(b.begin(), b.end());
		la=(int)a.length();
		lb=(int)b.length();
		
		if (la < lb) {
			swap(la, lb);
			swap(a, b);
		}
		
		for (int i = 0; i<la-lb; i++) b+='0';
		
		for (int i=0; i<la; i++) {
			tmp = (a[i]-'0') + (b[i] - '0') + (up?1:0);
			if (tmp>=10) {
				s += '0'+tmp%10;
				up = true;
			} else {
				s += '0'+tmp;
				up = false;
			}
		}
		
		if (up)
			s+='1';
		reverse(s.begin(), s.end());
		if (s.length()>80)
			cout << "overflow" << endl;
		else
			cout << s << 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++) {
		vector<int> v1(101), v2(101);
		string str1, str2;
		cin >> str1 >> str2;
		for (int j = 0; j < str1.size(); j++) v1[j] = str1[str1.size() - j - 1] - '0';
		for (int j = 0; j < str2.size(); j++) v2[j] = str2[str2.size() - j - 1] - '0';
		for (int j = 0; j < 100;j++) {
			v1[j] += v2[j];
			if (v1[j] >= 10) {
				v1[j] -= 10;
				v1[j + 1]++;
			}
		}
		int dig = 0;
		for (int j = 100; j >= 0; j--) {
			if (v1[j] != 0) {
				dig = j; break;
			}
		}
		if (dig >= 80) cout << "overflow" << endl;
		else {
			for (int j = dig; j >= 0; j--) {
				cout << v1[j];
			}
			cout << endl;
		}
	}
}

C++: #include<string>
#include<vector>
#include<iostream>
#include<algorithm>
#include<cstdio>
#include<cstdlib>
#include<sstream>
#include<functional>
#include<map>
#include<stack>
#include<set>
#include<cmath>

#define REP(i,s,e) for(int i=int(s);i<int(e);i++)
#define rep(i,n) for(int i=0;i<int(n);i++)

#define EPS 1e-6

using namespace std;

int main(){

	int n;
	cin >> n;

	rep(i,n){

	string a,b;
	cin >> a >> b;

	int sa=a.size(); int sb=b.size(); int p=max(sa,sb);
	vector<char>s;

	if(sa>80 || sb>80)
	cout << "overflow" << endl;
	else{

	string z;
	if(sa>sb){
	rep(j,sa-sb)
	z+='0';
	b=z+b;
	}
	if(sb>sa){
	rep(j,sb-sa)
	z+='0';
	a=z+a;
	}

    int c=0;
	rep(j,p){

	c+=(a[p-1-j]-'0')+(b[p-1-j]-'0');

	s.push_back(c%10+'0');
	c=c/10;

	}

    if(c==1) s.push_back(c%10+'0');

    if(s.size()<=80){
    rep(k,s.size()) cout << s[s.size()-1-k];
    cout << endl;
    }
    else
    cout << "overflow" << endl;

    s.clear();

	}

	}

	return 0;
}

C++: #include <iostream>
#include <cstring>
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<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,g;
    int k,l;
    bool check;

    cin >> n;
    rep(i,n){
        char c1[81], c2[81];
        int i1[81] = {0}, i2[81] = {0};
        int ans[82] = {0};
        check = false;
        k = 0; l = 0;
        cin >> c1 >> c2;
        for(int j = 80; 0 <= j; j--){
            if(j <= strlen(c1) - 1){
                i1[k++] = c1[j] - '0';
            }
            if(j <= strlen(c2) - 1){
                i2[l++] = c2[j] - '0';
            }
        }
        rep(j, 80){
            ans[j]+= i1[j] + i2[j];
            if(ans[j] >= 10){
                ans[j]-=10;
                ans[j+1]++;
            }
        }
        if(ans[80] != 0){
            cout << "overflow";
        }else{
            for(int j = 81; 0 <= j; j--){
                if(ans[j] != 0){
                    check = true;
                }
                if(check == true)
                    cout << ans[j];
            }
        }
        cout << endl;
    }
}