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 <string.h>
 
int main(void) {
    char a[200],b[200],result[200],temp[200];
    int ketaagari;
    int i,j;
    int max;
    char in;
    int overflow;
    int count,kazu;
    scanf("%d",&kazu);
    for(count=0;count<kazu;count++) {
        for(i=0;i<200;i++) {
            a[i]=0;b[i]=0;
        }
        i=0;
        overflow=0;
    	
    	scanf(" %s ",temp);
    	
    	i=strlen(temp);
    	if(i>80)overflow=1;
        for(j=0;j<i;j++)a[j]=temp[i-j-1]-'0';
    	scanf(" %s ",temp);
    	i=strlen(temp);
    	if(i>80)overflow=1;

    	for(j=0;j<i;j++)b[j]=temp[i-j-1]-'0';
 
        for(i=0,ketaagari=0,max=0;i<80;i++) {
            result[i]=(a[i]+b[i]+ketaagari)%10;
            ketaagari=(a[i]+b[i]+ketaagari)/10;
            if(result[i]!=0)max=i;
        }
        if(ketaagari)overflow=1;

    	if(overflow==0) {
            for(i=max;i>=0;i--)printf("%d",result[i]);
            printf("\n");
        } else printf("overflow\n");
    }
    return 0;
}

C++: #include <iostream>
#include <string>
#define MAX 80

using namespace std;

int main()
{
	int n;
	cin >> n;
	for (int counter = 0; counter < n; counter++){
		string a, b;
		cin >> a >> b;
		if (a.length() > MAX || b.length() > MAX){
			cout << "overflow" << endl;
			continue;
		}
		int inta[MAX + 2], intb[MAX + 2], sum[MAX + 2];
		fill(inta, inta + MAX + 2, 0);
		fill(intb, intb + MAX + 2, 0);
		fill(sum, sum + MAX + 2, 0);
		for (int i = 0; i < (signed)a.length(); i++){
			inta[i] = a[a.length() - i - 1] - '0';
		}
		for (int i = 0; i < (signed)b.length(); i++){
			intb[i] = b[b.length() - i - 1] - '0';
		}

		int over = 0;
		for (int i = 0; i < MAX + 2; i++){
			sum[i] = inta[i] + intb[i] + over;
			if (sum[i] >= 10){
				sum[i] %= 10;
				over = 1;
			}
			else{
				over = 0;
			}
		}

		for (int i = MAX + 1;; i--){
			if (sum[i] != 0){
				if (i >= MAX){
					cout << "overflow" << endl;
					break;
				}
				for (int j = i; j >= 0; j--){
					cout << sum[j];
				}
				cout << endl;
				break;
			}
		}
	}
	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)-1;
        c[cp]='\0';
        t=0;
        while((t||ap||bp)&&cp>0){
            t+=(ap?a[--ap]-'0':0)+(bp?b[--bp]-'0':0);
            c[--cp]='0'+(t%10);
            t/=10;
        }
        if(t)printf("overflow\n");
        else printf("%s\n",&c[cp]);
    }
    return 0;
}

C++: #include <iostream>
#include <string>
#define MAX 80

using namespace std;

int main()
{
	int n;
	cin >> n;
	for (int counter = 0; counter < n; counter++){
		string a, b;
		cin >> a >> b;
		if (a.length() > MAX || b.length() > MAX){
			cout << "overflow" << endl;
			continue;
		}
		if (a.length() == 1 && b.length() == 1 && a[0] == 0 && b[0] == 0){
			cout << 0 << endl;
			continue;
		}
		int inta[MAX + 2], intb[MAX + 2], sum[MAX + 2];
		fill(inta, inta + MAX + 2, 0);
		fill(intb, intb + MAX + 2, 0);
		fill(sum, sum + MAX + 2, 0);
		for (int i = 0; i < (signed)a.length(); i++){
			inta[i] = a[a.length() - i - 1] - '0';
		}
		for (int i = 0; i < (signed)b.length(); i++){
			intb[i] = b[b.length() - i - 1] - '0';
		}

		int over = 0;
		for (int i = 0; i < MAX + 2; i++){
			sum[i] = inta[i] + intb[i] + over;
			if (sum[i] >= 10){
				sum[i] %= 10;
				over = 1;
			}
			else{
				over = 0;
			}
		}

		for (int i = MAX + 1;; i--){
			if (sum[i] != 0){
				if (i >= MAX){
					cout << "overflow" << endl;
					break;
				}
				for (int j = i; j >= 0; j--){
					cout << sum[j];
				}
				cout << endl;
				break;
			}
		}
	}
	return 0;
}

C++: #include<iostream>
#include<string>
#include<vector>
int main(){
	std::string a,b;
	std::cin>>a>>b;
	std::string ans;
	if(a.size()<10&&b.size()<10){
		int A=0;
		int B=0;
		for(int i=a.size()-1;i>=0;i--)A+=a[i]-'0';
		for(int i=b.size()-1;i>=0;i--)B+=b[i]-'0';
		std::cout<<A+B<<std::endl;
	}else{
		int kr=0;
		int m=a.size()<=b.size()?a.size():b.size();
		int M=a.size()>b.size()?a.size():b.size();
		for(int i=m-1;i>=0;i--){
			int c=a[M],d=b[m];
			if(c+d>10)kr=1;
			int e=c+d;
			if(kr==1)e-=10;
			ans.push_back(e);
			a.erase(a.back()),b.erase(b.back());
			m--,M--;
		}
		ans.append(a);
	}
	std::cout<<ans<<std::endl;
	return 0;
}

C++: #include<iostream>
#include<string>
typedef unsigned int u;
using namespace std;
typedef string::iterator s;
u z=100000000;
void p(u*a,u*l,u n){
	if(a-l)
	p(a+1,l,(n+=*a*10)/z),
	*a=n%z;	
}
u t(u*a,u*l){
	u r=0;
	char b[11]={};
	if(a-l)
	if(!t(a+1,l)){
		if(*a)cout<<*a,r=1;
	}else
		sprintf(b,"%d",*a+z),
		cout<<b+1,
		r=1;
	return r;
}
void d(u*a,u*l,u*b,u n=0){
	if(a-l)
	d(a+1,l,b+1,(n+=(*a+*b))/z),
	*a=n%z;
}
int main(){
	u n;
	cin>>n;
	string a,b;
	getline(cin,a);
	for(;n--;){
		getline(cin,a);
		getline(cin,b);
		u x[11]={},y[11]={},c=0;
		for(s i=a.begin();i!=a.end();p(x,x+10,*i++-48))
			if(++c>80)goto g;
		c=0;
		for(s i=b.begin();i!=b.end();p(y,y+10,*i++-48))
			if(++c>80)goto g;
		d(x,x+11,y);
		if(x[10])goto g;
		else if(!t(x,x+10))cout<<'0';
		if(0)
g:
		cout<<"overflow";
		cout<<'\n';
	}
}

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;

	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)
    cout << "overflow" << endl;
    else
    rep(k,s.size()) cout << s[s.size()-1-k];

    cout << endl;
    s.clear();

	}

	return 0;
}