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>
using namespace std;

int main() {

  int n;
  cin >> n;

  string c = "0123456789";

  for ( int i = 0; i < n; i++ ) {

    string input1,input2;
    cin >> input1 >> input2;

    string ans = "";

    int k = 0;
    for ( int j = 0; j < 80; j++ ) {

      int x = input1.size()-1-j;
      if ( x >= 0 ) {
	k += input1[x] - '0';
      }
      int y = input2.size()-1-j;
      if ( y >= 0 ) {
	k += input2[y] - '0';
      }
      ans = c[k%10] + ans;
      k /= 10;

    }

    if ( k != 0 ) {
      cout << "overflow" << endl;
    }else {
      bool flag = true;
      for ( int i = 0; i < ans.size(); i++ ) {
	if ( ans[i] == '0' && flag ) continue;
	flag = false;
	cout << ans[i];
      }
      if ( flag ) cout << "0";
      cout << endl;
    }

  }

  return 0;

}

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

int chomp(char *str){
    int len = strlen(str);
    if(str[len-1]=='\n'){
        str[len-1] = '\0';
        len--;
    }
    return len;
}

int main(){
    int n;
    int i,j;
    int s1,s2,la,sm;
    char a1[102],a2[102],*pla,*psm,dummy[2];
    int carry,r;
    scanf("%d",&n);
    fgets(dummy,2,stdin);
    for(i=0;i<n;i++){
        fgets(a1,101,stdin);
        fgets(a2,101,stdin);
        s1 = chomp(a1);
        s2 = chomp(a2);
        if(s1>80||s2>80){
            printf("overflow\n");
            goto NEXT;
        }
        if(s1<s2){
            pla = a2;
            la = s2;
            psm = a1;
            sm = s1;
        }else{
            pla = a1;
            la = s1;
            psm = a2;
            sm = s2;
        }
        carry = 0;
        for(j=0;j<la;j++){
            r = (j<sm) ? (psm[sm-j-1]-'0') : 0;
            r += (pla[la-j-1]-'0')+carry;
            carry = r/10;
            pla[la-j] = '0' + r%10;
        }
        if(carry){
            pla[0] = '1';
        }else{
            pla = (s1<s2) ? &a2[1] : &a1[1];
        }
        if(j+carry>80){
                printf("overflow\n");
                goto NEXT;
        }
        printf("%s\n",pla);
NEXT:
        printf("");
    }
}

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

using namespace std;


int main(){

	int n;
	cin >> n;
	for(int i=0 ; i<n ; i++){
		string a, b;
		cin >> a;
		cin >> b;

		if(b.size() > a.size()){
			string temp;
			temp = a;
			a = b;
			b = temp;
		}

		if(a.size() >= 80){
			cout << "overflow" << endl;
		}else{
			for(int j=0 ; j<b.size() ; j++){
				a[a.size()-1-j] = a[a.size()-1-j] + b[b.size()-1-j] -'0';
				if(a[a.size()-1-j] > '9'){
					a[a.size()-1-j] -= 10;
					for(int l=0 ; l<a.size()-1 ; l++){
						a[a.size()-2-l] ++;
						if(a[a.size()-2-l] > '9'){
							if(a.size()-2-l == 0){
								a = "0" + a;
							}
							a[a.size()-2-l] -= 10;
						}else break;
					}
				}
			}
			if(a.size() >= 80){
				cout << "overflow" << endl;
			}else{
				cout << a << 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;
				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;
				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{
				if((c[m - i -1]) || f){
					printf("%d", c[m - i - 1]);
					f = 1;
				}
			}
		}
		printf("\n");
	}
	return 0;
}

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

using namespace std;

string reverse(const string& str){
  string ret;
  string::const_reverse_iterator it = str.rbegin();
  for(; it != str.rend(); it++) {
    ret.push_back(*it);
  }
  return ret;
}

void cut0(string& str){
  while(str[str.size() - 1] == '0'){
    str.erase(str.size() - 1);
  }
}

int main () {
  int n;
  string a, b, sum;
  
  cin >> n;
  while(n--) {
    bool overflow = false;

    cin >> a >> b;
    a = reverse(a);
    b = reverse(b);
    a.resize(80, '0');
    b.resize(80, '0');
    sum.clear();
    sum.resize(80, '0');
    
    for(size_t i = 0; i < 80; i++) {
      sum[i] += a[i] - '0' + b[i] - '0';
      if(sum[i] > '9'){
        sum[i] -= 10;
        if(i < 79) {
          sum[i+1] = '1';
        }
        else {
          overflow = true;
          break;
        }
      }
    }
    if(overflow){
      cout << "overflow" << endl;
    }
    else {
      cut0(sum);
      cout << reverse(sum) << endl;
    }
  }
}

C++: #include<iostream>
#include<string>
#include<cstdio>
#include<algorithm>
#include<stack>
#include<queue>
#include<vector>
#include<cmath>
#include<utility>
#include<set>
#include<complex>
#define vi vector<int>
#define vvi vector<vector<int> >
#define ll long long int
#define vl vector<ll>
#define vvl vector<vector<ll>>
#define vb vector<bool>
#define vc vector<char>
#define vs vector<string>
#define ld long double
#define INF 1e9
#define EPS 0.0000000001
#define rep(i,n) for(int i=0;i<n;i++)
#define CC puts("-------ok--------");
#define all(in) in.begin(), in.end()
#define bv vector<bool>
using namespace std;
typedef pair<int, int>PA;
using namespace std;
#define MAX  999999
int main (){
    int n; cin >> n;
    rep(k,n){
        string s; string s2;
        cin >> s >> s2;
        vi v(100,0);
        if(s.length()>=80 || s2.length() >= 80){
            cout << "overflow" << endl;
        }else{
            vi v(100,0); vi v2(100,0);
            vi ans(100,0);
            if(s.length()<s2.length()) swap(s,s2);
            rep(i,s.length())
                v[i] = (int)s[i]-48;
            rep(i,s2.length())
                v2[i] = (int)s2[i]-48;
            int num = (int)max(s2.length(),s.length());
            reverse(v.begin(), v.begin()+s.length());
            reverse(v2.begin(),v2.begin()+ s2.length());
            int ansize = 0;
            rep(i,num){
                ansize++;
                ans[i] += v[i]+v2[i];
                if(ans[i] >= 10){
                    ans[i]-=10;
                    ans[i+1]+=1;
                    if(i == num-1)
                        ansize++;
                }
            }
            reverse(ans.begin(), ans.begin()+ansize);
            rep(i,ansize)
            cout << ans[i];
            cout << endl;
        }
        
        }
        return 0;
}