Go: package main

import (
	"bufio"
	"fmt"
	"math/big"
	"os"
	"strconv"
)

var sc = bufio.NewScanner(os.Stdin)

func main() {
	sc.Scan()
	dataset, _ := strconv.Atoi(sc.Text())
	var firstdata *big.Int

	for i := 0; i < dataset*2; i++ {
		n := new(big.Int)
		sc.Scan()
		data, _ := n.SetString(sc.Text(), 10)

		if i%2 == 0 {
			firstdata = data
			continue
		}

		result := new(big.Int)
		if result := result.Add(data, firstdata); len(result.String()) > 80 {
			fmt.Println("overflow")
			continue
		} else {
			fmt.Println(result)
		}
	}
}

C++: #include<iostream>
#include<string>
#define max(a,b) a>b?a:b
using namespace std;
int che(char a){
	char i;
	int j=0;
	for(i='0';i<='9';i++){
		if(a==i){
			return j;
		}
		j++;
	}
	return 0;
}
char cha(int a){
	char i;
	int j=0;
	for(i='0';i<='9';i++){
		if(j==a){
			return i;
		}
		j++;
	}
}
int main(){
	string a,b;
	int c=0;
	int i,j,k,m,n;
	int x,y;
	char d[200];
	cin>>n;
	for(k=0;k<n;k++){
		cin>>a>>b;
		x=a.size();
		y=b.size();
		m=max(x,y);
		for(i=1;i<=m;i++){
			j=che(a[x-i])+che(b[y-i])+c;
			if(j>=10){
				c=1;
				j-=10;
			}else{
				c=0;
			}
			d[i]=cha(j);
		}
		if(c==1){
			d[i]=cha(1);
			c=0;
			i++;
		}
		cout<<i<<endl;
		if(i>81){
			cout<<"overflow"<<endl;
			continue;
		}
		for(j=i-1;j>0;j--){
			cout<<d[j];
		}
		cout<<endl;
	}
	return 0;
}

C++: #include<cstdio>

int main(){
	int a[82] = {};
	int b[82] = {};
	int c[82] = {};
	int n;
	int k,l,m,f;
	char t;
	int of;

	scanf("%d\n", &n);


	for(int i = 0; i < n; i++){
		of = k = l = 0;
		scanf("%c", &t);
		while(t - 10){
			k++;
			a[k] = t - '0';
			scanf("%c", &t);
			if(k == 81){
				of = 1;
				scanf("%c", &t);
				while(t - 10){
					scanf("%c", &t);
				}
			}
		}
		scanf("%c", &t); 

		while(t - 10){
			l++;
			b[l] = t - '0';
			scanf("%c", &t);
			if(l == 81){
				of = 1;
				scanf("%c", &t);
				while(t - 10){
					scanf("%c", &t);
				}
			}

		}

		m = 0;
		f = 0;

		while(k - m){
			c[m] = a[k - m];
			m++;
		}
		m = 0;
		while(l - m){
			c[m] += b[l - m ] + f;
			f = 0;
			if(c[m] > 9){
				c[m] -= 10;
				f = 1;
			}
			m++;
		}

		while(f){
			c[m] += 1;
			f = 0;
			if(c[m] > 9){
				c[m] -= 10;
				f = 1;
			}
			m++;
		}

		if(k > m){
			m = k;
		}

		for(int i = 0; i < m; i++){
			if(m == 81 || of){
				printf("overflow");
				i = m;
			}else{
				printf("%d", c[m - i - 1]);
			}
		}
		printf("\n");
	}
	return 0;
}

C++: #include <cstdio>
#include <cmath>
#include <cstdlib>
#include <cstring>

using namespace std;

const int MAX_INPUT = 1000000;
const int ARRAY_SIZE = 11;
const int MAX_LEN = 8;
char inputA[MAX_INPUT];
char inputB[MAX_INPUT];

void toBigInt(char *str, long *r) {
    int len = strlen(str);
    int endIndex = ARRAY_SIZE - (len / MAX_LEN) - 1;

    {
        int offset = 0;
        for (int i = 0; i < ARRAY_SIZE; ++i) {
            char temp[MAX_LEN + 1] = { 0 };
            int readLen = ((len - offset) < MAX_LEN) ? len - offset : MAX_LEN;
            char* start = str + len - offset - readLen;

            strncpy(temp, start, readLen);
            r[ARRAY_SIZE - i- 1] = (long)atoi(temp);
            offset += readLen;
        }
    }
}

int main() {
    const char OVER[] = "overflow\n";
    int n = 0;
    scanf("%d", &n);

    for (int i = 0; i < n; ++i) {
        scanf("%s\n%s", &inputA, &inputB);
        long a[ARRAY_SIZE] = { 0 };
        long b[ARRAY_SIZE] = { 0 };
        long r[12] = { 0 };
        int lenA = 0;
        int lenB = 0;
        const int MAX_INPUT_LEN = 80;
        lenA = strlen(&inputA[0]);
        lenB = strlen(&inputB[0]);
        if ((MAX_INPUT_LEN < lenA) || (MAX_INPUT_LEN < lenB)) {
            printf(OVER);
            continue;
        }

        toBigInt(inputA, a);
        toBigInt(inputB, b);
        for (int i = 0; i < ARRAY_SIZE; ++i) {
            int curRIndex = 12 - i - 1;
            r[curRIndex] += a[ARRAY_SIZE - i - 1] + b[ARRAY_SIZE - i - 1];
            const int MAX = 99999999;
            if (MAX < r[curRIndex]) {
                r[curRIndex - 1] = r[curRIndex] / (MAX + 1);
                r[curRIndex] %= (MAX + 1);
            }
        }

        if (9999999 < r[2]) {
            printf(OVER);
            continue;
        }

        {
            bool started = false;
            bool noOutput = true;
            for (int i = 0; i < 12; ++i) {
                if (!started && (0 < r[i])) {
                    started = true;
                    printf("%d", r[i]);
                    noOutput = false;
                    continue;
                }

                if (started) {
                    long temp = (r[i] == 0) ? 1 : r[i];
                    for (int j = 0; temp * pow(10, j) < 10000000; ++j) {
                        printf("0");
                    }

                    printf("%d", r[i]);
                }
            }

            if (noOutput) {
                printf("0");
            }
        }
        printf("\n");

        for (int i = 0; i < MAX_INPUT; ++i) {
            inputA[i] = 0;
            inputB[i] = 0;
        }
    }

    return 0;
}

C++: //include
//------------------------------------------
#include <bits/stdc++.h>

using namespace std;

////typedef
////------------------------------------------
//typedef vector<int> vi;
//typedef vector<vi> vii;
//typedef vector<string> vs;
//typedef vector<bool> vb;
//typedef vector<vb> vbb;
//typedef pair<int, int> pii;
//typedef long long ll;
//typedef unsigned long long ull;
//
////container util
////------------------------------------------
//#define all(a)  (a).begin(),(a).end()
//#define rall(a) (a).rbegin(), (a).rend()
//#define pb push_back
//#define mp make_pair
//#define iter(i,c) for(auto i=(c).begin(); i!=(c).end(); ++i)
//
////repetition
////------------------------------------------
//#define loop(i,a,b) for(int i=(a);i<(b);++i)
//#define rep(i,n)  loop(i,0,n)
//
////constant
////--------------------------------------------
//const double eps = 1e-10;
//const double pi  = acos(-1.0);
//const double inf = (int)1e8;
//
//// output vector
//// ------------------------------------------
//namespace std {
//    template<typename T> ostream & operator<<(ostream& os, vector<T> const& v){
//        for(auto it=v.begin(); it!=v.end(); ++it){
//            os << *it << (it+1==v.end() ? "" : ",");
//        }
//        return os;
//    }
//}
//
////clear memory
//#define clr(a) memset((a), 0 ,sizeof(a))

string IntToString(int number) {
    stringstream ss;
    ss << number;
    return ss.str();
}

int main(){
    int n;
    cin >> n;
    for(int p=0; p < n; p++){
        string ans;
        string s, t;
        cin >> s >> t;
        if(s.size() > t.size()) swap(s, t);


        int q = t.size()-s.size();
        for(int i=0; i < q; i++) s = "0" + s;

        int up = 0;
        for(int i=s.size()-1; 0 <= i; i--){
            int d = atoi(s.substr(i, 1).c_str());
            int f = atoi(t.substr(i, 1).c_str());
            ans = IntToString((d+f+up)%10) + ans;

            if(0 <= d+f+up-10) up = 1;
            else up = 0;
        }
        if(up == 1) ans = IntToString(1) + ans;
        cout << ans << endl;
     }
}