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<stdio.h>
#include<string.h>
#include<iostream>

int count(int data[]){
	int cnt = 0,i=0;
	while(1){
		if(data[i] == 10)break;
		cnt++;
		i++;
	}
	return cnt;
}

void sum(char data1[],char data2[],int ans[]){
	int i;
	for(i = 0;i < strlen(data1) || i < strlen(data2);i++){
		int st1,st2,st3;
		if(i >= strlen(data1)){
			st1 = 0;
		}else{
			st1 = data1[strlen(data1)-i-1];
			st1 = st1 - 48;
		}
		if(i >= strlen(data2)){
			st2 = 0;
		}else{
			st2 = data2[strlen(data2)-i-1];
			st2 = st2 - 48;
		}
		st3 = st1 + st2;
		if(st3 >= 10){
			if(ans[i] == 1){
				ans[i] = st3 - 9;
				ans[i + 1] = 1;
			}else{
				ans[i] = st3 - 10;
				ans[i + 1] = 1;
			}
		}else{
			if(ans[i] == 1){
				if(st3 == 9){
					ans[i] = 0;
					ans[i + 1] = 1;
				}else{
					ans[i] = st3 + 1;
				}
			}else{
				ans[i] = st3;
			}
		}
	}
	if(ans[i] == 1){
		ans[i + 1] = 10;
	}else{
		ans[i] = 10;
	}
}

void reverse(int ans[]){
	if(count(ans) > 80){
		std::cout << "overflow" << std::endl;
	}else{
		int i;
		for(i = count(ans)-1;i >= 0;i--){
			printf("%d",ans[i]);
		}
	}
}

int main(void){
	int cnt,i;
	scanf("%d",&cnt);
	for(i = 0;i < cnt;i++){
		int ans[82];
		char data1[82],data2[82];
		scanf("%s",data1);
		scanf("%s",data2);
		if(strlen(data1) > 80 || strlen(data2) > 80){
			std::cout << "overflow" << std::endl;
		}else{
			sum(data1,data2,ans);
			reverse(ans);
		}
		printf("\n");
	}
	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[10000],se[10000],ans[10000];

	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 <vector>
    #include <string>
    #include <sstream>
     
    class BigInteger {
    	// use 18 digit a element
    	std::vector<uint64_t> splited_num_;
    public:
    	BigInteger(std::string str) {
    		set_data(str);
    	}
    	BigInteger(std::vector<uint64_t>&& splited_num) {
    		splited_num_ = std::move(splited_num);
    	}
    	BigInteger operator+(const BigInteger& rhs) {
    		return BigInteger(plus(splited_num_, rhs.get_data()));
    	}
    	std::vector<uint64_t> get_data() const {return splited_num_;}
    	std::string to_string() const {
    		std::string ret("");
    		const int size = splited_num_.size();
    		for (int i = 0; i < size; ++i) {
    			const uint64_t& n = splited_num_[size - i - 1];
    			ret += n == 0 ? "000000000000000000" : std::to_string(n);
    		}
    		return ret;
    	}
    private:
    	void set_data(const std::string& str) {
    		std::string sub;
    		int length = str.length();
    		while (length >= 18) {
    			int pos = length - 18;
    			sub = str.substr(pos, 18);
    			splited_num_.push_back(to_uint64(sub));
    			length -= 18;
    		}
    		if (length > 0) {
    			sub = str.substr(0, length);
    			splited_num_.push_back(to_uint64(sub));
    		}
		}
    	std::vector<uint64_t> plus(std::vector<uint64_t> lhs,
    						       std::vector<uint64_t> rhs) const {
    		std::vector<uint64_t> ret;
     		uint64_t carry = 0;
     		
     		const int max = std::max(lhs.size(), rhs.size());
     		const int min = std::min(lhs.size(), rhs.size());
     		// ensure that lhs.sizs() is larger than rhs.size()
     		if (lhs.size() < rhs.size()) {
     			std::swap(lhs, rhs);
     		}
     		// fit container size
     		for (int i = 0; i < (max - min); ++i) {
     			rhs.push_back(0);
     		}
     		
     		for (int i = 0; i < max; ++i) {
     			uint64_t sum = lhs.at(i) + rhs.at(i) + carry;
     			carry = sum / 1000000000000000000;
     			sum = sum % 1000000000000000000;
     			ret.push_back(sum);
     		}
     		if (carry > 0) {
     			ret.push_back(carry);
     		}
     		return ret;
    	}
    	uint64_t to_uint64(const std::string& str) const {
    		std::istringstream iss(str);
    		uint64_t ret = 0;
    		iss >> ret;
    		return ret;
    	}
    };
     
    int main() {
    	std::vector<BigInteger> ans_list;
    	int n;
    	std::cin >> n;
    	for (int i = 0; i < n; ++i) {
    		std::string str1, str2;
    		std::cin >> str1;
    		std::cin >> str2;
    		BigInteger bi1(str1);
    		BigInteger bi2(str2);
    		ans_list.push_back(bi1 + bi2);
    	}
    	
    	for (auto ans : ans_list) {
    		auto ans_str = ans.to_string();
    		std::cout << (ans_str.length() > 80 ? "overflow" : ans_str) << std::endl;
    	}
    }

C++: #include <iostream>
#include <queue>
#include <stack>
#include <vector>
#include <stdio.h>
#include <algorithm>
#include <string.h>
#include <string>
#include <cmath>
#include <complex>
#include <map>
using namespace std;

#define rep(i,n) for(int i=0;i<int(n);++i)
#define ALL(v) (v).begin(),(v).end()
#define PB push_back
#define EPS 1e-8
#define F first
#define S second

static const double PI=6*asin(0.5);
typedef pair<double,double> P;
typedef complex<double> CP;
typedef long long ll;


int a[1000],b[1000],c[1000];

int main(){
	int n;
	cin>>n;
	rep(i,n){
		memset(a,0,sizeof(a));
		memset(b,0,sizeof(b));
		memset(c,0,sizeof(c));
		string s,s2;
		cin>>s;
		//cout<<s<<endl;
		//cout<<s.size()<<endl;
		if(s.size()>80){
			cout<<"overflow\n";
			cin>>s2;
			continue;
		}
		rep(j,s.size()){
			a[90-j]=s[s.size()-j-1]-'0';
			//cout<<s[s.size()-j-1]-'0';
		}
		//cout<<a[87]<<endl;
		cin>>s2;
		if(s2.size()>80){
			cout<<"overflow\n";
			continue;
		}
		rep(j,s2.size()){
			b[90-j]=s2[s2.size()-j-1]-'0';
		}
		/*
		rep(j,91) cout<<a[j];
		cout<<endl;
		rep(j,91) cout<<b[j];
		cout<<endl;
		*/
		int t=0;
		for(int j=90;j>=0;j--){
			c[j]=(a[j]+b[j]+t)%10;
			if((a[j]+b[j]+t)>9) t=1;
			else t=0;
		}
		bool f=true;
		/*
		rep(j,91) cout<<c[j];
		cout<<endl;
		*/
		rep(j,91){
			if(f&&c[j]!=0){
				f=false;
				if(j<=10){
					cout<<"overflow";
					break;
				}
			}
			if(!f) cout<<c[j];
		}
		cout<<endl;
		
	}
	return 0;
}

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

using namespace std;

string reverse(string s) {
	string ans = "";
	int i = s.size()-1;
	for (int j = 0; i >= 0; i--, j++)
		ans += s[i];
	return ans;
}

string add(string n, string m) {
	string ans;
	int a, carry = 0;
	if (n.size() < m.size())
		swap(n, m);
	int i = n.size();
	int j = m.size();
	for (; i > 0; i--) {
		a = (n[i-1] - '0') + carry;
		if (j) {
			a = a + (m[j-1] - '0');
			j--;
		}
		if (a > 9) {
			a -= 10;
			carry = 1;
		} else {
			carry = 0;
		}
		stringstream ss;
		ss << a;
		string t = ss.str();
		ans += t[0];
	}
	if (carry)
		ans += "1";
	return reverse(ans);
}

int main() {
	string n, m, ans;
	int N;
	cin >> N;
	for (int i = 0; i < N; i++) {
		cin >> n >> m;
		ans = add(n, m);
		if (ans.size() > 80)
			cout << "overflow" << endl;
		else
			cout << ans << endl;
	}

	return 0;
}