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 <cassert>
#include <iostream>
#include <string>
#include <vector>

typedef std::vector<char> TLongInt;
TLongInt Decode(const std::string& Code)
{
    std::vector<char> Value(100, 0);
    for(int i = 0; i < Code.size(); ++i){
        Value[i] = Code[Code.size() -1 -i] - '0';
    }
    return Value;
}
TLongInt Add(const TLongInt& A, const TLongInt& B)
{
    assert(A.size() == B.size());
    TLongInt C(A.size(), 0);
    int kuriagari = 0;
    for(int i = 0; i < A.size(); ++i){
        int V = A[i] + B[i] + kuriagari;
        C[i] = V % 10;
        kuriagari = V / 10;
    }
    return C;
}

void Print(const TLongInt& Value, std::ostream& out)
{
    bool Left = true;
    for(int i = Value.size() - 1; i >= 0; --i){
        if(Value[i] == 0 && Left) continue;
        if(i >= 80){
            out << "overflow" << std::endl;
            return;
        }
        Left = false;
        out << int(Value[i]);
    }
    if(Left) out << "0";
    out << std::endl;
}

int main()
{
    int N;
    std::cin >> N;
    for(int i = 0; i < N; ++i){
        std::string first, second;
        std::cin >> first;
        std::cin >> second;
        Print(Add(Decode(first), Decode(second)), std::cout);
    }
    return 0;
}

C++: #include<cstdio>
#include<iostream>
int main(){
    int n;
    scanf("%d",&n);
    for(int i=0;i<n;i++){
        char moneywrite[2][100];
        int money[2][100];
        int stl[2]={0,0};
        int stlstl;
        int sum[81];
        scanf("%s",moneywrite[0]);
        scanf("%s",moneywrite[1]);
        for(int j=0;moneywrite[0][j]!='\0';j++){
            money[0][j]=moneywrite[0][j]-'0';
            stl[0]++;
        }
        for(int j=0;moneywrite[1][j]!='\0';j++){
            money[1][j]=moneywrite[1][j]-'0';
            stl[1]++;
        }
        if(stl[0]>stl[1]){
            stlstl=stl[0];
            int p;
            p=stl[0]-stl[1];
            for(int j=0;j<stl[0];j++){
                money[1][stl[0]-j]=money[1][stl[0]-j-p];
            }
            for(int k=0;k<p;k++)
                money[1][k]=0;
        }
        else if(stl[1]>stl[0]){
            stlstl=stl[1];
            int p;
            p=stl[1]-stl[0];
            for(int j=0;j<stl[1];j++){
                money[0][stl[1]-j]=money[0][stl[1]-j-p];
            }
            for(int k=0;k<p;k++)
                money[0][k]=0;
        }
        else{
            stlstl=stl[1];
        }
/*
        for(int j=0;j<stlstl;j++)
            printf("[%d][%d]\n",money[0][j],money[1][j]);
*/
        if(80<=stlstl){
            printf("overflow");
            goto exit;
        }
        for(int j=stlstl-1;0<=j;j--){
            sum[j]=money[0][j]+money[1][j];
            if(10<=sum[j]){
                if(stlstl==79){
                    printf("overflow");
                    goto exit;
                }
                else if(j==0){
                    printf("1");
                    sum[j]%=10;
                }
                else{
                    money[0][j-1]++;
                    sum[j]%=10;
                }
            }
        }
        for(int j=0;j<stlstl;j++)
            printf("%d",sum[j]);
exit:
        puts("");
    }
    return 0;
}

C++: #include<string>
#include<cstdio>
#include<cstdlib>
#include<cstring>
#include<complex>
#include<cmath>
#include<algorithm>
#include<map>
#include<vector>
#include<sstream>
#include<iostream>
using namespace std;
string str_plus(string s,string t){
    reverse(s.begin(),s.end());
    reverse(t.begin(),t.end());

    string u = "0",v;
    int n1 = s.size(),n2 = t.size();
    int k = max(n1,n2);
    while(s.size()!=k)s+='0';
    while(t.size()!=k)t+='0';
    for(int i = 0;i < k ;i++){
        u[i] += s[i] - '0' + t[i] - '0';
        if(u[i]>='9')u+="1",u[i]-=10;
        else u+="0";
    }
    reverse(u.begin(),u.end());
    bool flg = true;
    for(int i = 0;i < u.size();i++){
        if(u[i]=='0'&flg)continue;
        v+=u[i];
        flg = false;
    }
    return v;
}
int main(void){

    int n;
    cin>>n;
    while(n--){
        string s,t;
        cin>>s>>t;
        string u = str_plus(s,t);
        if(u.length()>=80)cout<<"overflow"<<endl;
        else cout<<u<<endl;
    }


    return 0;
}

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

using namespace std;

int main(){
    
    string d[100000];
    string e[100000];
    
    stack<char> st1;
    stack<char> st2;
    stack<int> st3;
    int n=0;
    scanf("%d",&n);
    for(int i=0; i<n; i++){
        cin>>d[i];
        cin>>e[i];
    }
    
    for(int i=0; i<n; i++){
        //cin>>d;
        //cin>>e;
        
        for(int j=0; j<(int)(d[i].size()); j++){
            st1.push(d[i][j]);
        }
        for(int j=0; j<(int)(e[i].size()); j++){
            st2.push(e[i][j]);
        }
        int temp=0;
        char temp_char;
        while(st1.empty()==false||st2.empty()==false){
            if(st1.empty()==false&&st2.empty()==false){
                temp_char=st1.top();
                temp+=atoi(&temp_char);
                temp_char=st2.top();
                temp+=atoi(&temp_char);
                st1.pop();
                st2.pop();
            }else if(st1.empty()==false&&st2.empty()==true){
                temp_char=st1.top();
                temp+=atoi(&temp_char);
                st1.pop();
            }else if(st1.empty()==true&&st2.empty()==false){
                temp_char=st2.top();
                temp+=atoi(&temp_char);
                st2.pop();
            }
            st3.push(temp%10);
            temp=temp/10;
        }
        
        if(temp!=0){
            st3.push(temp);
        }
        
        if(st3.size()>80){
            printf("overflow");
        }else{
            while(st3.empty()==false){
                printf("%d",st3.top());
                st3.pop();
            }
        }
        printf("\n");
        
        
    }
    
}