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 <string>
#include <list>
using namespace std;

class bigdata
{
public:
  list < int >data;
  string to_str ()
  {
    string str (data.size (), '0');
    int i = 0;
    auto it = data.begin ();
    while (it != data.end ())
      {
	string buf = to_string (*it);
	  str[i] = buf[0];
	  it++;
	  i++;
      }
    return str;
  }
  bigdata & operator= (string str)
  {
    for (int i = 0; i < str.length (); i++)
      data.push_back (stoi (str.substr (i, 1)));
  }
  bigdata & operator+ (bigdata & obj)
  {
    if (this->data.size () < obj.data.size ())
      {
	int buf = obj.data.size () - this->data.size ();
	for (int i = 1; i <= buf; i++)
	  this->data.push_front (0);
      }
    if (this->data.size () > obj.data.size ())
      {
	int buf = this->data.size () - obj.data.size ();
	for (int i = 1; i <= buf; i++)
	  obj.data.push_front (0);
      }
    this->data.push_front (0);
    obj.data.push_front (0);

    auto it_a = this->data.begin ();
    auto it_b = obj.data.begin ();

    while (it_a != this->data.end ())
      {
	*it_a += *it_b;
	auto bufit = it_a;
	for (int i = 0; i < this->data.size () - 1; i++)
	  {
	    if (*bufit >= 10)
	      {
		bufit--;
		*bufit += 1;
		++bufit;
		*bufit -= 10;
		bufit--;
	      }
	  }
	it_a++;
	it_b++;
      }
    it_a = this->data.begin ();
    if (*it_a == 0)
      this->data.pop_front ();
    return *this;
  }
};

int
main ()
{
  string s1, s2;
  while (cin >> s1 >> s2)
    {
      bigdata data1, data2;
      data1 = s1;
      data2 = s2;
      data1 = data1 + data2;
      cout << data1.to_str () << endl;
    }
}

C++: #include <iostream>
#include <string>
#include <utility>
#include <algorithm>
#include <queue>
#include <map>
#include <fstream>
#include <array>
#include <utility>
#include <functional>
#include <stack>

using namespace std;
typedef long long Int;
#define REP(i , x) for(int i = 0 ; i < x ; ++i)

typedef pair<int, int> P;


int main()
{
	int n;
	string a, b;
	cin >> n;
	REP(i, n){
		int a_a[85] = { 0 };
		int b_a[85] = { 0 };
		int c_a[85] = { 0 };
			cin >> a >> b;
			if (a.length() >= 80 || b.length() >= 80){
				cout << "overflow" << endl;
			}

			REP(i, a.length()){
				a_a[i] = a[a.length() - i -1] - '0';
			}
			
			REP(i, b.length()){
				b_a[i] = b[b.length() - i -1] - '0';
			}

			REP(i, 84){
				int temp = a_a[i] + b_a[i] + c_a[i];
				c_a[i + 1] += temp / 10;
				c_a[i] = temp % 10;
			}
			int i = 84;
			while (!c_a[i])--i;
			if (i >= 80){
				cout << "overflow" << endl;
			}
			while (i >= 0){
				cout << c_a[i];
				--i;
			}
			cout << endl;
		}
	return 0;
}

C++: #include <iostream>
#include <cstring>
#include <cstdio>
using namespace std;
int main(){
  int n;
  char a[101];
  char b[101];
  int temp;
  int p = 0;
  int i;
  int flag;
  cin >> n;
  while(p < n){
    flag = 0;
    for(i = 0;i < 101;i++){
      a[i] = '0';
      b[i] = '0';
    }
    scanf("%s %s",a,b);
    temp = 100;
    for(i = strlen(a) - 1;i >= 0;i--){
      a[temp] = a[i];
      a[i] = '0';
      temp--;
    }
    temp = 100;
    for(i = strlen(b) - 1;i >= 0;i--){
      b[temp] = b[i];
      b[i] = '0';
      temp--;
    }
    for(i = 100;i >= 0;i--){
      a[i] += b[i] - '0';
      if(a[i] > '9'){
	a[i - 1]++;
	a[i] -= 10;
      }
    }
    for(i = 0;i < 101;i++){
      if(a[i] >= '1'){
	if(i <= 20){
	  cout << "overflow";
	  break;
	}
	flag = 1;
      }
      if(flag || i == 100){
	cout << a[i];
      }
    }
    cout << endl;
    p++;
  }
  return(0);
}

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

using namespace std;

int main(){
	int n,l1,l2,i;
	string a, b;
	char *ans;
	int ia,ib,sum;
	cin>>n;
	while(n--){
		cin>>a>>b;
		if(a.length()>80 || b.length()>80){
			cout<<"overflow"<<endl;
			continue;
		}
		if(a.length()==80 || b.length()==80){
			if((int)a[0]-'0'+(int)b[0]-'0'>9){
				cout<<"overflow"<<endl;
				continue;
			}
		}
		if(a.length()<b.length()){
			swap(a,b);
		}
		l1=a.length();
		l2=b.length();
		ans=new char[l1+1];
		for(i=0;i<l1;i++)ans[i]='0';
		for(i=1;i<l2+1;i++){
			ia=(int)a[l1-i]-(int)'0';
			ib=(int)b[l2-i]-(int)'0';
			sum=ia+ib;
			ans[l1-i]+=sum;
		}
		for(i=l2+1;i<l1+1;i++){
			ans[l1-i]=a[l1-i];
		}
		for(i=0;i<l1;i++){
			if(ans[i]-'0'>9){
				ans[i]=ans[i]-10;
				ans[i-1]+=1;
			}
		}
		for(i=0;i<l1;i++){
			cout<<(int)ans[i]-'0';
		}
		cout<<endl;
	}
	return 0;
}