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 <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(itr,c) 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, n;

  cin >> n;
  while(n--){
    string s1, s2, tmp, res;

    cin >> s1;
    if((int)s1.size() > 80){
      cout << "overflow" << endl;
      continue;
    }
    cin >> s2;
    if((int)s2.size() > 80){
      cout << "overflow" << endl;
      continue;
    }

    vector<int> a(100,0);
    rep(i,s1.size()) a[i] = s1[(int)s1.size()-1-i] - '0';
    vector<int> b(100,0);
    rep(i,s2.size()) b[i] = s2[(int)s2.size()-1-i] - '0';

    rep(i,100){
      int sum = a[i] + b[i];
      if(sum >= 10){ sum %= 10; a[i+1]++;}
      tmp += sum + '0';
    }

    reverse(tmp.begin(), tmp.end());
    bool f = false;
    rep(i,tmp.size()){
      if(tmp[i] != '0') f = true;
      if(f) res += tmp[i];
    }

    if((int)res.size() > 80) cout << "overflow" << endl;
    else cout << res << endl;
  }

  return 0;
}

C++: #include <iostream>
#include <string>
#include <vector>
#include <cmath>
#include <algorithm>
#include <cstdlib>
#include <ctime>
#include <stdio.h>

using namespace std;

int main()
{
	int i,j;
	string s1,s2;

	cin>>s1>>s2;
	
	if(s1.size()<s2.size())swap(s1,s2);

	for(i=0;i<s2.size();i++){s1[i+s1.size()-s2.size()]+=s2[i]-'0';}

	for(i=s1.size()-1;i>0;i--){
		if(s1[i]>'9'){s1[i]-=10; s1[i-1]++;}
	}

	if(s1[0]>'9'){s1[0]-=10;cout<<"1";}

	cout<<s1<<endl;

	return 0;
}

C++: #include<bits/stdc++.h>
using namespace std;


string solve(string s1, string s2){

while(s1.size()<85){
s1 = '0' + s1;
}
while(s2.size()<85){
s2 = '0' + s2;
}
string ret = "";
int kuri = 0;
for(int i=0;i<80;i++){
int r = (int)(s1[s1.size()-1-i]-'0' + s2[s2.size()-1-i]-'0') + kuri;
kuri = r/10;
r = r%10;
ret = ((char)('0'+r)) + ret;
}
if (kuri>0) return "overflow"; else return ret;
}

int main(void){
int N;
cin >> N;
for(int i=0i<N;i++){
string s1, s2;
cin >> s1 >> s2;
cout << solve(s1, s2) << endl;
}
return 0;
}

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

int main(){
	int n;
	cin >> n;
	while( n-- > 0 ){
		string num1, num2;
		cin >> num1 >> num2;
		int sum[101];
		int degit = 0;
		for( int i = 0; i < 101; i++ ) sum[i] = 0;
		for( int i = 0; i < num1.size(); i++ ){
			sum[100-i] = num1[num1.size()-i-1]-'0';
		}
		int d;
		for( int i = 0; i < num1.size() || i < num2.size() || degit != 0; i++ ){
			int now;
			if( i < num2.size() ){
				now = sum[100-i] + (int)num2[num2.size()-i-1]-(int)('0') + degit;
			}else{
				now = sum[100-i] + degit;
			}
			sum[100-i] = (now%10);
			if( now >= 10 ) degit = 1;
			else degit = 0;
			d = i;
		}
		if( d < 80 ){
			for( int i = 100-d; i < 101; i++ ) cout << sum[i];
			cout << endl;
		}else{
			cout << "overflow" << endl;
		}
	}
}

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

using namespace std;

int main() {

	int n;

	cin >> n;

	for (int i = 0; i < n; ++i) {
		bool carry = false;
		string first, second;
		cin >> first >> second;

		string sum;

		auto it1 = first.rbegin();
		auto it2 = second.rbegin();
		while (it1 != first.rend() || it2 != second.rend()) {

			int temp1 = (it1 != first.rend()) ? (*it1++) - '0' : 0;
			int temp2 = (it2 != second.rend()) ? (*it2++) - '0' : 0;
			int temp_sum = temp1 + temp2 + (carry ? 1 : 0);
			if (temp_sum / 10 != 0) {
				carry = true;
				temp_sum %= 10;
			}
			else {
				carry = false;
			}

			sum.insert(0, to_string(temp_sum));
		}
		if (carry == true) {
			sum.insert(0, 1, '1');
		}

		if (first.length() > 80 || second.length() > 80 || sum.length() > 80) {
			cout << "overflow" << endl;
		}
		else {
			cout << sum << endl;
		}
	}


	return 0;
}

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

int main(){
	const int size = 80;
	char str1[size + 10], str2[size + 10];
	int num1[size], num2[size], res[size];
	int n, leng1, leng2, loop, kuri;
	bool output;
	
	scanf("%d", &n);
	
	while(n--){
		fill(str1, str1 + size + 10, 0);
		fill(str2, str2 + size + 10, 0);
		fill(num1, num1 + size, 0);
		fill(num2, num2 + size, 0);
		fill(res, res + size, 0);
		scanf("%s", str1);
		scanf("%s", str2);
		
		//char -> int
		for(leng1 = 0; (int)str1[leng1 + 1] >= 48 && (int)str1[leng1 + 1] <= 57 && leng1 + 1 < size + 10; leng1++);//length of str1
		for(leng2 = 0; (int)str2[leng2 + 1] >= 48 && (int)str2[leng2 + 1] <= 57 && leng2 + 1 < size + 10; leng2++);//length of str2
		
		loop = 1;
		while(leng1 >= 0){
			num1[size - loop] = (int)str1[leng1] - 48;
			leng1--; loop++;
		}
		loop = 1;
		while(leng2 >= 0){
			num2[size - loop] = (int)str2[leng2] - 48;
			leng2--; loop++;
		}
		kuri = 0;
		for(int i = size - 1; i >= 0; i--){
			res[i] = num1[i] + num2[i] + kuri;
			kuri = res[i] / 10;
			if(kuri > 0) res[i] -= 10;
		}
		
		if(kuri > 0) printf("overflow\n");
		else{
			output = false;
			for(int i = 0; i < size; i++){
				if(!output && res[i] == 0) continue;
				else{
					output = true;
					printf("%d", res[i]);
				}
			}
			if(!output) printf("0");
			printf("\n");
		}
	}
	return 0;
}

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

int charToInt(char);
string sumStr(string, string);

int charToInt(char c) {
  return (int)c - (int)'0';
}

string sumStr(string a, string b) {
  int pa = a.length()-1, pb = b.length()-1;
  string str(81, '0');
  int pstr = str.length()-1;
  int next = 0;
  while(pa >= 0 && pb >= 0) {
    int temp = charToInt(a[pa]) + charToInt(b[pb]) + next;
    next = temp / 10;
    str[pstr] = (char)(temp%10+(int)'0');

    pa--;
    pb--;
    pstr--;
  }
  while(pa >= 0) {
    int temp = charToInt(a[pa]) + next;
    next = temp / 10;
    str[pstr] = (char)(temp%10+(int)'0');

    pa--;
    pstr--;
  }
  while(pb >= 0) {
    int temp = charToInt(b[pb]) + next;
    next = temp / 10;
    str[pstr] = (char)(temp%10+(int)'0');

    pb--;
    pstr--;
  }
  str[pstr] = (char)(next + (int)'0');

  for(int i = 0; i < str.length(); i++) {
    if(str[i] != '0') {
      return str.substr(i, str.length()-i);
    }
  }
  return "0";
}

int main(void) {
  int N;
  cin >> N;
  for(int i = 0; i < N; i++) {
    bool overflow = false;
    int data[N][8];
    string sum;
    for(int j = 0; j < 2; j++) {
      string str;
      cin >> str;

      if(!overflow) {
	if(j == 0) sum = str;
	else       sum = sumStr(sum, str);
        if(sum.length() > 80) overflow = true;
      }
    }

    if(overflow) cout << "overflow" << endl;
    else         cout << sum << endl;
  }
  return 0;
}