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>
#include <stack>
#include <algorithm>
using namespace std;

int main(){
	string s1, s2;
	stack<int> sum;
	int tmp, carry, n, len1, len2;

	cin>>n;
	while(n--){
		tmp = carry = 0;
		while(!sum.empty())sum.pop();
		cin>>s1>>s2;
		len1 = s1.length()-1;
		len2 = s2.length()-1;
		reverse(s1.begin(), s1.end());
		reverse(s2.begin(), s2.end());

		for(int i=0; i<81; i++){
			tmp = carry;
			if(i<=len1) tmp += s1[i]-'0';
			if(i<=len2) tmp += s2[i]-'0';
			sum.push(tmp%10);
			carry = tmp/10;
		}
		if(sum.top() != 0)cout<<"overflow\n";
		else{
			for(int i=0; i<80; i++){
				if(sum.top()==0)sum.pop();
				else break;
			}
			while(!sum.empty()){cout<<sum.top(); sum.pop();}
			cout<<"\n";
		}
	}	
	return 0;
}

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

using namespace std;

int main (void) {
    int N;
    cin >> N;
    while (N--) {
        string s1, s2;
        cin >> s1;
        cin >> s2;
        if (s1.size() > 80) {
            cout << "overflow" << endl;
            continue;
        }
        if (s2.size() > 80) {
            cout << "overflow" << endl;
            continue;
        }
        vector<int> n1, n2, res;
        for (int i = s1.size(); i--;) {
            n1.push_back((int)(s1[i] - '0'));
        }
        for (int i = s2.size(); i--;) {
            n2.push_back((int)(s2[i] - '0'));
        }
        if (n1.size() < n2.size()) {
            for (int i = n1.size(); i < n2.size(); i++) {
                n1.push_back(0);
            }
        } else if (n2.size() < n1.size()) {
            for (int i = n2.size(); i < n1.size(); i++) {
                n2.push_back(0);
            }
        }
        int s, c = 0;
        for (int i = 0; i < n1.size(); i++) {
            s = n1[i] + n2[i] + c;
            res.push_back(s % 10);
            c = s / 10;
        }
        if (c != 0) {
            res.push_back(c);
        }
        int size = res.size();
        while (size > 0 && res[size - 1] == 0) {
            size--;
        }
        if (size > 80) {
            cout << "overflow" << endl;
        } else {
            int i = size;
            while (i--) {
                cout << res[i];
            }
            cout << endl;
        }
    }
    return 0;
}

C++: #include<stdio.h>
#include<math.h>
#include<string.h>

int main(void){

	int N,math1,math2,kuri,keta;
	int i,j,k;
	char fi[100],se[100],ans[100];

	scanf("%d",&N);

	for(i=0;i<N;i++){

		scanf("%s",&fi);
		scanf("%s",&se);

		if(strlen(fi)<=80&&strlen(se)<=80){
		for(j=strlen(fi)-1,k=strlen(se)-1,keta=0,kuri=0;j>=0||k>=0;j--,k--,keta++){
			if(j<0) math1=0;
			else math1=fi[j]-'0';
			if(k<0) math2=0;
			else math2=se[k]-'0';
            /*ans[keta]=((math1+math2+kuri)%10)+'0';*/
			kuri=(math1+math2+kuri)/10;
		}
		if(kuri==1) printf("1");
		if(keta-1<=80){
			for(j=keta-1;j>=0;j--){
				printf("%c",ans[j]);
			}
		}
		else printf("overflow");
		}
		else printf("overflow");
		printf("\n");
	}

	return 0;
}

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;
  int loop;
  cin >> loop;
  while (loop-- > 0)
    {
      cin >> s1 >> s2;
      bigdata data1, data2;
      data1 = s1;
      data2 = s2;
      data1 = data1 + data2;
      if ((data1.to_str().length()) > 80)
	cout << "overflow" << endl;
      else
	cout << data1.to_str () << endl;
    }
}

C++: #include<iostream>
#include<stdio.h>
#include <algorithm>
#include<string.h>
#include <utility>
using namespace std;
int main(){
	int m;
	string in,in2;
	int a[1000],b[1000],ans[1000];
	int f;
	int alen,blen,mlen;
	cin>>m;
	while(m--){
		for(int i=0;i<90;i++){
			a[i]=b[i]=ans[i]=0;
		}
		cin>>in;
		cin>>in2;
		alen=in.size();
		blen=in2.size();
		for(int i=0;i<alen;i++){
			a[alen-1-i]=in[i]-'0';
		}
		for(int i=0;i<blen;i++){
			b[blen-1-i]=in[i]-'0';
		}
		mlen=max(alen,blen);
		if(mlen>80){
			cout<<"overflow"<<endl;
			continue;
		}
		f=0;
		for(int i=0;i<mlen+1;i++){
			ans[i]=(a[i]+b[i]+f)%10;
			f=(a[i]+b[i]+f)/10;
		}
		if(ans[mlen]!=0){
			//if(mlen>79){
				//cout<<"overflow"<<endl;
				//continue;
			//}
			//cout<<ans[mlen];
		}
		
		for(int i=0;i<mlen;i++){
			cout<<ans[mlen-1-i];
		}cout<<endl;
		
		
	}
	return 0;
}

C++: #include <stdio.h>

#define BUF_SIZE 256

int main(void) {
	char buf[BUF_SIZE];
	int num1[81];
	int num2[81];
	int sum[81];
	char str[82];
	int n;
	int i, j, k;
	
	fgets(buf, sizeof(buf), stdin);
	sscanf(buf, "%d", &n);
	
	for (i=0; i<n; i++) {
		for (j=0; j<81; j++) {
			num1[j] = num2[j] = 0;
		}
		for (j=0; j<81; j++) {
			sum[j]=0;
		}		
		
		fgets(buf, sizeof(buf), stdin);
		sscanf(buf, "%s", &str);
		j=k=0;
		while (str[j] != '\0')  j++;
		while (--j >= 0) num1[k++] = (int)(str[j] - '0'); 
		
		fgets(buf, sizeof(buf), stdin);
		sscanf(buf, "%s", &str);
		j=k=0;
		while (str[j] != '\0')  j++;
		while (--j >= 0) num2[k++] = (int)(str[j] - '0'); 
		
		for (j=0; j<80; j++) {
			sum[j] += num1[j]+num2[j];
			if (sum[j] >= 10) {
				sum[j+1] += 1;
				sum[j] = sum[j]%10;
			}
		}
		
		if (sum[80] != 0) {
			printf("overflow\n");
		}
		else {
			j=79;
			while (sum[j] == 0)  j--;
			for (; j>=0; j--) {
				printf("%d", sum[j]);
			}
			printf("\n");
		}
	}
	
	return 0;
}