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++: /* package whatever; // don't place package name! */

import java.util.*;
import java.lang.*;
import java.io.*;
import java.math.*;

/* Name of the class has to be "Main" only if the class is public. */
public class Main
{
	public static void main (String[] args) throws java.lang.Exception
	{
		Scanner sc = new Scanner(System.in);
		int q = sc.nextInt();
 		for(int i = 0; i < q; ++i){
 			BigInteger n1 = sc.nextBigInteger();
 			BigInteger n2 = sc.nextBigInteger();
 			BigInteger ans = n1.add(n2);
 			String sa = ans.toString();
 			if(sa.length() > 80){
 				System.out.println("overflow");	
 			}
 			else{
 				System.out.println(sa);
 			}
 		}	
	}
}

C++: #include <cstdio>
#include <cstdlib>
#include <cstring>
#include <cmath>
#include <cassert>
#include <iostream>
#include <sstream>
#include <string>
#include <vector>
#include <queue>
#include <set>
#include <map>
#include <utility>
#include <numeric>
#include <algorithm>
#include <bitset>
#include <complex>
#include <fstream>

using namespace std;

typedef unsigned uint;
typedef long long Int;
typedef vector<int> vint;
typedef pair<int,int> pint;
#define mp make_pair
#define MSG(a) cout << #a << " = " << a << endl;
#define rep(i, n) for(int i = 0; i < n; i++)
#define All(v) v.begin(), v.end()

template<class T> void pv(T a, T b) { for (T i = a; i != b; ++i) cout << *i << " "; cout << endl; }
template<class T> void chmin(T &t, T f) { if (t > f) t = f; }
template<class T> void chmax(T &t, T f) { if (t < f) t = f; }

int main()
{
	int n;
	cin >> n;
	rep(i, n)
	{
		int ans[128];
		memset(ans, 0, sizeof(ans));
		string a, b;
		cin >> a >> b;

		int ai = a.size()-1, bi = b.size()-1;
		for(int i = 0; ai >= 0 || bi >= 0; ai--, bi--, i++)
		{
			int as = (ai >= 0 ? a[ai] - '0' : 0), bs = (bi >= 0 ? b[bi] - '0' : 0);

			ans[i] += as + bs;
			ans[i+1] += max(0, ans[i] - 9);
			if(9 < ans[i]) ans[i] = 0;

			if(i == 100) break;
		}

		if(ans[80] != 0 ||(80 < a.size() || 80 < b.size()))
		{
			cout << "overflow" << endl;
			continue;
		}
		else
		{
			int index = 0;
			rep(i, 120)
				if(ans[i] != 0)
					index = i;

			for(int i = index; i >= 0; i--)
				cout << ans[i];

			cout << endl;
		}
	}
}

C++: #include <iostream>
#include <string>
#include <cstdlib>
#define x 81
using namespace std;

int main(){
	int n;
	cin >> n;
	cin.ignore();
	for(int i=0;i<n;i++){
		string a,b;
		getline(cin,a);
		getline(cin,b);
		bool c = false;
		if(a.size()>=x) c = true;
		if(b.size()>=x) c = true;
		int p[x];
		int k = 0;
		for(int j=0;j<x;j++){
			p[j] = k;
			int s = a.size()-j-1;
			int t = b.size()-j-1;
			string f;
			string g;
			if(s>=0) f = a.substr(s,1);
			if(t>=0) g = b.substr(t,1);
			if(s>=0) p[j] += atoi(f.c_str());
			if(t>=0) p[j] += atoi(g.c_str());
			if(p[j]>=10){ p[j] -= 10; k = 1; }
			else k = 0;
			if(k*j==79) c = true;
		}
		if(c) cout << "overflow" << endl;
		else{
			int d = x-1;
			while(p[d]==0){ d--; }
			for(int j=d;j>=0;j--) cout << p[j];
			cout << endl;
		}
	}
}

C++: #include <cstdio>
#include <cstdlib>
#include <iostream>
#include <algorithm>
#include <cstring>
#include <string.h>
#include <queue>
#include <stack>
#include <utility>
#include <queue>
#define mp make_pair
#define pii pair<int,int> 
#define ff first
#define ss second

using namespace std;

int t,temp1,temp2,tempc;
char a[1000],b[1000];
int c[1000];

int addition(int n1,int n2)
{
	if(n1>=n2)
	{
		if(n2!=-1)
		{
			temp1=(int)a[n1]-48;
			temp2=(int)b[n2]-48;
			tempc=temp1+temp2;
			//printf("%d,%d,%d",temp1,temp2,tempc);
			if(tempc>=10)
			{
				tempc-=10;
				c[n1]+=1;
			}
			c[n1+1]+=tempc;
			//printf("%d,%d",c[n1+1],tempc);
			addition(n1-1,n2-1);
		}
		else if(n1!=-1)
		{
			tempc=(int)a[n1]-48;
			if(tempc+c[n1+1]>=10)
			{
				c[n1+1]-=10;
				c[n1]+=1;
			}
			c[n1+1]+=tempc;
			addition(n1-1,n2);
		}
	}
	else
	{
		addition(n2,n1);
	}
}

int main()
{
	scanf("%d",&t);
	while(t--)
	{
		scanf("%s",a);
		scanf("%s",b);
		for(int x=0;x<max(strlen(a),strlen(b))+5;x++)
		{
			c[x]=0;
		}
		if(strlen(a)>80||strlen(b)>80)
		{
			printf("overflow\n");
			continue;
		}
		else
		{
			addition(strlen(a)-1,strlen(b)-1);
			//printf("%d",(int)a[1]);
			//printf("%d,%d\n",strlen(a),strlen(b));
		}
		for(int z=0;z<max(strlen(a),strlen(b))+1;z++)
		{
			if(c[z]==0&&z==0)
			continue;
			printf("%d",c[z]);
		}
		printf("\n");
	}
	return 0;
}

C++: #include <iostream>
#include <string>
#include <vector>
#include <algorithm>
#include <cmath>
#include <cstdio>
#include <functional>
#include <numeric>
#include <stack>
#include <queue>
#include <map>
#include <set>
#include <utility>
#include <sstream>
#include <complex>

using namespace std;

#define FOR(i,a,b) for(long long i=(a);i<(b);i++)
#define REP(i,N) for(long long i=0;i<(N);i++)
#define ALL(s) (s).begin(),(s).end()
#define fi first
#define se second

#define PI acos(-1.0)
#define INF 1000000007

typedef long long ll;
typedef pair<ll, ll> P;
typedef pair<string, ll> PS;
typedef vector<ll> V;

int k = 0, n;
string s, t;
vector<string> ans;

int main(){
	cin >> n;
	REP(j,n){
		cin >> s >> t;
		k = 0;
		if (s.size() >= 80 || t.size() >= 80){
			cout << "overflow" << endl;
		}
		reverse(ALL(s));
		reverse(ALL(t));
		int a = max(s.size(), t.size()), b = min(s.size(), t.size());
		if (s.size() > t.size()){
			REP(i, a - b){
				t += "0";
			}
		}
		else{
			REP(i, a - b){
				s += "0";
			}
		}		
		reverse(ALL(s));
		reverse(ALL(t));
		REP(i, s.size()){
			int x = (s[s.size()-1-i] - '0') + (t[s.size()-1-i] - '0');
			string q;
			stringstream ss;
			if (i != s.size() - 1){
				ss << ((x + k) % 10);
			}
			else{
				ss << x + k;
			}
			ss >> q;
			ans.push_back(q);
			if (x + k >= 10){
				k = (x + k) / 10;
			}
			else{
				k = 0;
			}
		}
		reverse(ALL(ans));
		REP(i, ans.size()){
			cout << ans[i];
		}
		cout << endl;
		ans.clear();
	}
}

C++: #include<iostream>
#include<stdio.h>
#include <algorithm>
#include <utility>
using namespace std;
using namespace std;
int main(){
	
	int m;
	char in[2][10000];
	char ans[100000];
	int ie[2];
	int ien[2];
	bool max10;
	int Min;
	bool NMin;
	scanf("%d",&m);
	for(;m--;){
		
		for(int i=0;i<2;i++){
			scanf(" %s",&in[i]);
		}
		for(int i=0;i<2;i++){
			for(ie[i]=0;in[i][ie[i]]!='\0';ie[i]++){
				in[i][ie[i]]=in[i][ie[i]]-'0';
			}
		}
		ie[0]--;
		ie[1]--;
		Min=max(ie[0],ie[1]);
		if((Min>79)||((Min>78)&&(ie[0]==ie[1])&&((int(in[0][0])+int(in[1][0]))>9))){
			cout<<"overflow"<<endl;
		}else{	
			if(ie[0]>ie[1]){
				NMin=0;
			}else{
				NMin=1;
			}
			ien[0]=ie[0];
			ien[1]=ie[1];
			for(;min(ien[0],ien[1])+1;ien[0]--,ien[1]--){
				in[NMin][ien[NMin]]=in[0][ien[0]]+in[1][ien[1]];
				if(in[NMin][ien[NMin]+1]>9){
					in[NMin][ien[NMin]+1]-=10;
					in[NMin][ien[NMin]]  +=1;
				}
			}
			max10=0;
			
			for(;ien[NMin];ien[NMin]--){
				in[NMin][ien[NMin]]=in[NMin][ien[NMin]];
				if(in[NMin][ien[NMin]+1]>9){
					in[NMin][ien[NMin]+1]-=10;
					in[NMin][ien[NMin]]  +=1;
				}
			}
			for(int i=0;i<2;i++){
				for(int j=0;j<ie[i]+1;j++){
					in[i][j]=in[i][j]+'0';
				}
			}
			if(max10) cout<<"1";
			cout<<in[NMin]<<endl;
		}
	}
	return 0;
}