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 <cstdio>
#include <cstdlib>
#include <cmath>
#include <map>
#include <utility>
#include <set>
#include <iostream>
#include <string>
#include <vector>
#include <algorithm>
#include <sstream>
#include <queue>
#include <cstring>
#include <functional>

using namespace std;

#define REP(i,a,b) for((i)=(a);(i)<(int)(b);(i)++)
#define rep(i,n) REP(i,0,n)
// BEGIN CUT HERE
//#define foreach(c,itr) for(typeof((c).begin()) itr=(c).begin();itr!=(c).end();itr++)
//template<class T> inline string toString(T x){ostringstream sout;sout<<x;return sout.str();}
// END CUT HERE

int main(){
  int i, j, n;

  scanf("%d",&n);
  rep(i,n){
    string str1, str2, res;
    cin >> str1;
    cin >> str2;

    vector<int> a(82, 0), b(82, 0);
    rep(j,str1.length()){
      a[j] = str1[str1.length()-1-j] - '0';
      b[j] = str2[str2.length()-1-j] - '0';
    }

    rep(j,81){
      int sum = a[j] + b[j];
      if(sum >= 10){ sum %= 10; a[j+1]++;}
      res += (sum + '0');
    }

    reverse(res.begin(), res.end());
    bool f = false;
    rep(j,res.length()){
      if(res[j] != '0') f = true;
      if(f) cout << res[j];
    }
    cout << endl;
  }

  return 0;
}

C++: #include <iostream>
#include <vector>
#include <string>
using namespace std;

int main(){
	int N;
	string str, str2;
	int ans_size, up, num;
	vector<int> ans;
	
	cin >> N;
	for(int n = 0; n < N; n++){
		cin >> str >> str2;
		
		up = 0;
		for(int i = 0; ; i++){
			if(i >= str.size() && i >= str2.size()){
				if(up > 0) ans.push_back(up);
				break;
			}
			num = 0;
			if(i < str.size()) num += str[str.size() - 1 - i] - 48;
			if(i < str2.size()) num += str2[str2.size() - 1 - i] - 48;
			ans.push_back((num + up) % 10);
			up = (num + up) / 10;
		}
		
		if(ans.size() > 80){
			cout << "overflow";
		}
		else{
			for(int i = ans.size() - 1; i >= 0; i--){
				cout << ans[i];
			}
		}
		cout << endl;
		ans.clear();
	}
	
	return 0;
}

C++: #include <iostream>
#include <vector>
#include <string>
#include <algorithm>
using namespace std;

#define loop(i,a,b) for(int i=(a); i<(int)(b); i++)
#define rep(i,b) loop(i,0,b)
#define all(c) (c).begin(), (c).end()
 
typedef vector<int> vi;
typedef vector<vi> vvi;
typedef pair<int,int> pii;

int main() {
  int n; cin >> n;
  while(n--) {
    string s1, s2; cin >> s1 >> s2;
      int m = max(s1.size(), s2.size());
      while(m != s1.size()) s1 = '0' + s1;
      while(m != s2.size()) s2 = '0' + s2;
      string ans;
      int c = 0;
      for(int i = m-1; i >= 0; i--) {
        int sum = s1[i]-'0' + s2[i]-'0' + c;
        c = sum/10;
        sum %= 10;
        ans += sum + '0';
      }
      if(c == 1) ans += '1';
      reverse(all(ans));
      cout << ans << endl;
  }
}

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 startIndex = ARRAY_SIZE - (len / MAX_LEN) - 1;

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

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

int main() {
    while (scanf("%s\n%s", &inputA, &inputB) != EOF) {
        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("overflow\n");
            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);
            }
        }

        {
            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");
    }

    return 0;
}