Go: package main

import (
        "fmt"
        "bufio"
        "os"
)

func main() {
        var n int
        var a, b string
        sc := bufio.NewScanner(os.Stdin)
        fmt.Scanf("%d", &n)
        for i := 0; i < n; i++ {
                sc.Scan()
                a = sc.Text()
                sc.Scan()
                b = sc.Text()

                if len(a) > len(b) {
                        s := add(a, b)
                        if s[len(a)] > 0 {
                                if len(a) >= 80 {
                                        fmt.Printf("overflow")
                                } else {
                                        for i := 0; i <= len(a);i++ {
                                                fmt.Printf("%d", s[len(a)-i])
                                        }
                                }
                                fmt.Printf("\n")
                        } else {
                                if len(a)-1 >= 80 {
                                        fmt.Printf("overflow")
                                } else {
                                        for i := 0; i < len(a);i++ {
                                                fmt.Printf("%d", s[len(a)-i-1])
                                        }
                                }
                                fmt.Printf("\n")
                        }
                } else {
                        s := add(b, a)
                        if s[len(b)] > 0 {
                                if len(b) >= 80 {
                                        fmt.Printf("overflow")
                                } else {
                                        for i := 0; i <= len(b);i++ {
                                                fmt.Printf("%d", s[len(b)-i])
                                        }
                                }
                                fmt.Printf("\n")
                        } else {
                                if len(b)-1 >= 80 {
                                        fmt.Printf("overflow")
                                } else {
                                        for i := 0; i < len(b);i++ {
                                                fmt.Printf("%d", s[len(b)-i-1])
                                        }
                                }
                                fmt.Printf("\n")
                        }
                }
        }
}

// expects ln > lm
func add(n string, m string) (s [100]int) {
        ln := len(n)
        lm := len(m)
        var sum [100]int

        for i := 0; i < ln; i++ {
                var s, t int
                s = int(n[ln-1-i]) - int('0')

                if lm - 1 - i >= 0 {
                        t = int(m[lm-1-i]) - int('0')
                } else {
                        t = 0
                }

                sum[i+1] = (s + t + sum[i]) / 10
                sum[i] = (s + t + sum[i]) % 10
        }

        return sum
}

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

#define CAPACITY 9

 
namespace stoi{
 
    int stoi(std::string str){
        int ret;
        std:: stringstream ss;
        ss << str;
        ss >> ret;
        return ret;
    }
 
}
class OverDigit{
 
public:
 
    int num_a[CAPACITY];
    int num_b[CAPACITY];
    OverDigit(std::string str_a, std::string str_b);
    void ChangeDigit(std::string str,int num[]);
    void Add();
};
 
OverDigit::OverDigit(std::string str_a,std::string str_b){
 
    ChangeDigit(str_a, num_a); 
    ChangeDigit(str_b, num_b);
 
}
 
void OverDigit::ChangeDigit(std::string str, int num[]){
 
    std::vector <int> num_str(CAPACITY, 0);
    int loc = CAPACITY, k = 8;
    int str_Max, str_rest;
 
    str_Max = str.length() / CAPACITY;//
    str_rest = str.length() % CAPACITY;
 
        for (int j = str.length() - CAPACITY; str_Max > 0; j = j-CAPACITY){
        num_str[k] = stoi::stoi(str.substr(j, CAPACITY));
        loc = k;
        k--;
        str_Max--;
    }
     
        if(str_rest!=0)num_str[loc - 1] = stoi::stoi(str.substr(0, str_rest));
 
    for (int j = 0; j < CAPACITY; j++){
    num[j]=num_str[j];
    }
}
//80?????????§?????¶???????
void OverDigit::Add(){
 
    int num_ans[CAPACITY] = {};
    int fat,overflow=0,count=0;
 
    for (int j = CAPACITY-1; j >0; j--){
        if (num_a[j] + num_b[j]<=999999999){
            num_ans[j] = num_a[j] + num_b[j];
        }
        else if (num_a[j] + num_b[j] > 999999999){
            fat = (num_a[j] + num_b[j]) - 1000000000;
            num_ans[j] = fat;
            num_a[j - 1]++;     
        }
 
    }
 
     if (num_a[0] + num_b[0] > 99999999){
        overflow = 1;
     }
     else{
         num_ans[0]= num_a[0] + num_b[0];
          
 
     }
 
    if (overflow == 1){
        std::cout << "overflow" << std::endl;
    }
    else {
        for (int j = 0; j < CAPACITY; j++){
            if (num_ans[j] != 0){
                count++;
                std::cout << num_ans[j];
 
            }
            else if (num_ans[j] == 0 && count >= 1){
                std::cout << "000000000";
            }
 
        }std::cout << std::endl;
    }
 
};
 
 
int main(){
    int N;
    std::string str_a, str_b;
 
    std::cin >> N;
    std::cin.ignore();
 
/*
   for (int i = 0; i < N; i++){
        getline(std::cin, str_a);
        getline(std::cin, str_b);
        OverDigit OverDigit_a(str_a,str_b);
       OverDigit_a.Add();
   }
*/
 
    return 0;
}

C++: 
#define _USE_MATH_DEFINES
#include <cfloat>
#include <climits>
#include <cmath>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <ctime>
#include <iostream>
#include <memory>
#include <string>

#include <algorithm>
#include <complex>
#include <list>
#include <map>
#include <numeric>
#include <queue>
#include <set>
#include <stack>
#include <vector>

using namespace std;

#ifdef _MSC_VER
#define _CRT_SECURE_NO_WARNINGS
#define __typeof__ decltype
#endif

#define ITER(c)         __typeof__((c).begin())
#define FOREACH(it, c)  for (ITER(c) it=(c).begin(); it != (c).end(); ++it)
#define RITER(c)        __typeof__((c).rbegin())
#define RFOREACH(it, c) for (RITER(c) it=(c).rbegin(); it != (c).rend(); ++it)
#define REP(i, n)       REPEAT(i, 0, n)
#define REPEAT(i, k, n) for (int i = (k); i < (n); ++i)
#define REVREP(i, n)    for (int i = ((n)-1); i >= 0; --i)
#define ESIZEOF(A)      (sizeof(A)/sizeof((A)[0]))
#define MIN(p1,p2)      (((p1)<(p2))?(p1):(p2))
#define MAX(p1,p2)      (((p1)>(p2))?(p1):(p2))
#define CIN_NO_SYNC     do { cin.tie(0); ios_base::sync_with_stdio(false); } while(0)

bool add(char *d, int n, string &s){
    int l = s.length();
    int j = 0;
    int carry = 0;
    while(carry || j < l){
        if(j >= n) return true;
        int dgt = (j < l) ? (s[l-j-1] - '0') : 0;
        d[j] = d[j] + dgt + carry;
        carry = d[j] / 10;
        d[j] %= 10;
        j++;
    }
    return false;
}
void print(const char *d, int n){
    bool pf = false;
    for(int i = n-1; i >= 0; i--){
        if(d[i] != 0 || pf){
            pf = true;
            printf("%c", '0' + d[i]);
        }
    }
    cout << endl;
}

int main(void){
    CIN_NO_SYNC;
    int n;
    cin >> n;
    char d[80];

    REP(i,n){
        string s1, s2;
        cin >> s1 >> s2;
        //cout << s1 << ":" << s2 << endl;
        memset(d, 0, sizeof(d));
        if(add(d, sizeof(d), s1) || add(d, sizeof(d), s2)){
            cout << "overflow" << endl;
        } else {
            print(d, sizeof(d));
        }
    }
    return 0;
}