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<algorithm>
using namespace std;
int main(){
  int n;
  cin>>n;
  cin.ignore();
  while(n--){
    string a,b;
    getline(cin,a);
    int as = a.size()-1;
    int bs = b.size()-1;
    int nmi = min(a.size(),b.size());
    int nma = max(a.size(),b.size());
    for(int x; i<nma; i++){
      if(i<nmi){x = tab+a[as-i]+b[bs-i]-'0'*2;}
      else if(i>as){x = tab+b[bs-i]-'0';}
      else if(i>bs){x = tab+a[as-i]-'0';}
      tab = x/10;
      num[i] = x%10;
    }
    if(tab){num[nma] = tab;i++;}
    if(i>80){cout <<"overflow"<<endl;}
    else{
      for(int x=i-1; x>=0; x--){
	cout <<num[x];
      }
      cout <<endl;
    }
  }
  return 0;
}

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

int main(){
	int n;
	cin >> n;
	for(int j=0;j<n;j++){
		char a[81],b[81],c[81];
		int ka = 0;
		int kb = 0;
		int kc = 0;
		bool ovf = false;
		cin >> a[ka];
		ka++;
		while(1){
			scanf("%c",&a[ka]);
			if(ka==80 and a[ka]!='\n') ovf = true;
			if(a[ka]=='\n' or ovf) break;
			ka++;
		}
		while(1){
			scanf("%c",&b[kb]);
			if(kb==80 and b[kb]!='\n') ovf = true;
			if(b[kb]=='\n' or ovf) break;
			kb++;
		}
		int k = 0;
		char t;
		for(int i=0;i<81;i++){
			if(i==80 and k==1) ovf = true;
			if((ka<=i and kb<=i) or ovf) break;
			int ia = 0;
			t = a[ka-i-1];
			if(ka>=i) ia = atoi(&t);
			int ib = 0;
			t = b[kb-i-1];
			if(kb>=i) ib = atoi(&t);
			int s = ia + ib + k;
			k = 0;
			if(s>9){ k = 1; s = s%10;}
			itoa(s,&c[i],10);
			kc++;
		}
		if(ovf) cout << "overflow" << endl;
		else{
			for(int i=kc-1;i>=0;i--) cout << c[i];
			cout << endl;
		}
	}
}

C++: #include <iterator>
#include <typeinfo>
#include <list>
#include <map>     
#include <numeric>
#include <utility>
#include <queue>
#include <set>
#include <sstream>
#include <iomanip>
#include <iostream>
#include <stack>
#include <string>
#include <vector>
using namespace std;
#define ll long long
#define ld long double
#define rep(a,t) for(int a=0;a<t;++a) 
#define forever while(true)
#define Sort(a) sort(a.begin(),a.end())
#define Reverse(a) reverse(a.begin(),a.end())
#define pb push_back
#define int_maxvalue numeric_limits<int>::max()
ll mod = 1e9 + 7;
string Addition(string a,string b) {
	string ans = "";
	int bef = 0;
	int be1 = 0;
	for (int i = a.length() - 1; i >= 0; i--) {
		int added = stoi(a.substr(i, 1)) + stoi(b.substr(i, 1))+be1;

		if (added >= 10) {
			bef = 1;
			added -= 10;
		}
		else {
			bef = 0;
		}
		stringstream ss;
		ss << added;

		ans = ss.str()+ans;
		if (i == 0) 
		{
			break;
		}
		be1 = bef;
	}
	if (be1 == 1) {
		ans = "1" + ans;
	}
	return ans;
}
int main() {
	int n;
	cin >> n;
	rep(o1, n) {
		string a, b;
		cin >> a >> b;
		if (a.length() < b.length()) {
			swap(a, b);
		}
		string b1 = b;
		for (int i = 0; i < a.length() - b1.length(); ++i) {
			b = '0' + b;
		}
		string ans = Addition(a, b);
		if (ans.length() > 80) { cout << "overflow" << endl; }
		else {
			cout << ans << endl;
		}
	}
	return 0;
}

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;

    int a[82], b[82];
    memset(a, 0, sizeof(a));
    memset(b, 0, sizeof(b));
    rep(j,str1.length()) a[j] = str1[str1.length()-1-j] - '0';
    rep(j,str2.length()) 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());
    int f = 0;
    rep(j,res.length()){
      if(res[j] != '0') f = 1;
      if(f) cout << res[j];
    }

    cout << endl;
  }

  return 0;
}

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

C++: #include <cstdio>
#include <cstring>

int main(){
 int n;
 scanf("%d\n",&n);
 for(int x = 0;x < n;x++){
  char A[101],B[101];
  scanf("%s\n%s\n",A,B);
  
  int l_a,l_b;
  if((l_a = strlen(A)) > 80 || (l_b = strlen(B)) > 80){
   printf("overflow\n");
   continue;
  }
  int val[100];
  memset(val,-1,sizeof(val));
  for(int i = 0;i < l_a || i < l_b;i++){
   val[i] = 0;
   if(l_a - i > 0) val[i] += A[l_a - i - 1] - '0';
   if(l_b - i > 0) val[i] += B[l_b - i - 1] - '0';
  }
  bool flg = false;
  int max;
  for(int j = 0;val[j] != -1;j++){
   max = j;
   if(val[j] >= 10){
    if(j + 1 >= 80) flg = true;
    else{
        val[j] -= 10;
        if(val[j+1] == -1) val[j+1] = 0;
        val[j + 1]++;
        max = j + 1;
    }
   }
  }
  if(flg || max >= 80){
   printf("overflow\n");
  }else{
   for(;max >= 0;max--){
    printf("%d",val[max]);
   }
   printf("\n");
  }
 }
 return 0;
}

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

int main()
{
  int N;
  char str1[81] = {};
  char str2[81] = {};
  char s1[81] = {};
  char s2[81] = {};
  cin >> N;
  for(int i = 0; i < N; i++){
    cin >> str1;
    cin >> str2;
    int m1 = strlen(str1), m2 = strlen(str2);
    if(m1 > 80 || m2 > 80)
      cout << "overflow" << endl;
    else if(atoi(str1) + atoi(str2) < 2147483647 && atoi(str1) + atoi(str2) > 0)
      cout << atoi(str1) + atoi(str2) << endl; 
    else{
      if(m1 > m2){
	for(int j = 0; j < m1 - m2 + 1; j++)
	  s1[j] = '0';
	s1[m1 - m2] = '\0';
	strcat(s1, str2);
	strcpy(s2, str1);
      }else if(m2 > m1){
	for(int k = 0; k < m2 - m1; k++)
	  s2[k] = '0';
	s2[m2 - m1] = '\0';
	strcat(s2, str1);
	strcpy(s1, str2);
      }else{
	strcpy(s1, str2);
	strcpy(s2, str1);
      }
      char ans[81] = {};
      int n = 0;
      for(int l = ((m1 > m2) ? m1 : m2) - 1; l >= 0; l--, n++)
	if((s1[l] - '0') + (s2[l] - '0') >= 10){
	  s1[l - 1] = s1[l - 1] + 1;
	  ans[l] = (s1[l]- '0') + (s2[l] - '0') + 38;
	}else
	  ans[l] = (s1[l] - '0') + (s2[l] - '0') + 48;
      ans[n] = '\0';
      if(n > 80)
	cout << "overflow" << endl;
      else{
	if((s1[0] - '0') + (s2[0] - '0') >= 10)
	  cout << "1";
	cout << ans << endl;
      }
    }
  }
}