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<vector>
using namespace std;
int main() {
	int n,i,j,k,s,t,u;
	int carry = 0;
	string a, b;
	vector<int> c;
	cin >> n;
	for (i = 0; i < n; i++) {
		c.clear();
		cin >> a;
		cin >> b;
		while (a.size() > b.size()) {
			b = "0" + b;
		}
		while (b.size() > a.size()) {
			a = "0" + a;
		}
		s = a.size();
		for (j = s-1; j >=0; j--) {
			t = a[j]-'0' + b[j]-'0';
			if (carry == 1) {
				carry = 0;
				t += 1;
			}
			while (t >= 10) {
				t -= 10;
				carry += 1;
			}
			c.push_back(t);
		}
		while (carry == 1) {
			carry = 0;
			c.push_back(1);
		}
		u = c.size();
		if (u > 80) {
			cout << "overflow" << endl;
		}
		for (k = u - 1; k >= 0; k--) {
			cout << c[k];
		}
		cout << endl;
	}
	return 0;
}

C++: #include <iostream>
#include <string>
#include <stdlib.h>
using namespace std;


int getValue(string str, int i){
	int endp,startp;

	endp = str.length() - 8 * i;
	startp = str.length() - 8 - 8 * i;
	if( endp <= 0 )
		return 0;
	else if( startp <= 0 ) {
		return atoi( str.substr(0,endp).c_str() );					
	}
	else {
		return atoi( str.substr(startp,8).c_str() );					
	}
	return -1;
}


int main()
{
	int num,i,j,startp,endp;
	string str1, str2;
	long v1,v2;
	int v[10];
	cin >> num;
	for(i = 0; i < num; i++ ) {
		cin >> str1;
		cin >> str2;
		if( str1.length() > 80 || str2.length() > 80 ){
			printf("overflow\n");
			continue;
		}
		//8桁ずつ分けて計算
		int flg = 0;
		for( j = 0; j < 10; j++ ) {
			v1 = getValue( str1, j );
			v2 = getValue( str2, j );
			
			v[j] = v1 + v2 + flg;
			if( v[j] >= 100000000 ) {
				flg = 1;
				v[j] -= 100000000;
			}
			else {
				flg = 0;
			}
		}
		if( flg == 1 )
			printf("overflow\n");
		else {
			flg = 0;
			for( j = 9 ; j >= 0; j-- ) {
				if( v[j] > 0 ){
					printf("%d",v[j] );
					flg = 1;
				}
				else if( flg == 1 ) {
					printf("%08d",v[j] );
				}
				
			}
			printf("\n");	
		}
	}




	return 0;
}

C++: #include<stdio.h>
#include<iostream>
#include<list>
#include<map>
#include<vector>
#include<algorithm>
#include<string.h>
#define range(i,a,b) for(int i = (a); i < (b); i++)
#define rep(i,b) range(i,0,b)
#define debug(x) cout << x << endl;
using namespace std;

int main(){
    int n,g;
    int k,l;
    bool check;

    cin >> n;
    rep(i,n){
        char c1[81], c2[81];
        int i1[81] = {0}, i2[81] = {0};
        int ans[82] = {0};
        check = false;
        k = 0; l = 0;
        cin >> c1 >> c2;
/*        for(int j = 80; 0 <= j; j--){
            if(j <= strlen(c1) - 1){
                i1[k] = c1[j] - '0';
                k++;
            }
            if(j <= strlen(c2) - 1){
                i2[l] = c2[j] - '0';
                l++;
            }*/
        }
        rep(j, 80){
            ans[j]+= i1[j] + i2[j];
            if(ans[j] >= 10){
                ans[j]-=10;
                ans[j+1]++;
            }
        }
        if(ans[80] != 0){
            cout << "overflow";
        }else{
            for(int j = 81; 0 <= j; j--){
                if(ans[j] != 0){
                    check = true;
                }
                if(check == true)
                    cout << ans[j];
            }
        }
        cout << endl;
    }
}

C++: #include<algorithm>
#include<iostream>
#include<cstdio>
#include<stdio.h>
#include<string>
#include<string.h>
#include"stdafx.h"
using namespace std;

int main()
{
	char a1,a2  ;
	long long a, b,c;
	int kaisu = 0;
	cin >> kaisu;
	for (int i = 0; i < kaisu; i++)
	{
		cin >> a;
		cin >> b;
		a1 =  GetDigit(a);
		a2 = GetDigit(a);
		if (a1 + a2 >= 80) { printf("overflow"); }
		else { c = a + b; printf(c + "\n"); }
	}
	return 0;
}
unsigned GetDigit(unsigned num) {
	return std::to_string(num).length();
}

C++: #include<iostream>
#include<stdio.h>
#include <algorithm>
#include<string.h>
#include <utility>
using namespace std;
using namespace std;
int main(){
	
	int m;
	char in[2][100];
	char ans[100];
	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]);
		}
		ie[0]=strlen(in[0])-1;
		ie[1]=strlen(in[1])-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(;ien[!NMin]+1;ien[0]--,ien[1]--){
				in[NMin][ien[NMin]]=in[0][ien[0]]+in[1][ien[1]]-'0';
				if(in[NMin][ien[NMin]]>9+'0'){
					in[NMin][ien[NMin]]-=10;
					in[NMin][ien[NMin]-1]  +=1;
				}
			}
			max10=0;
			for(;ien[NMin]>-1;ien[NMin]--){
				if(in[NMin][ien[NMin]]>9+'0'){
					in[NMin][ien[NMin]]-=10;
					in[NMin][ien[NMin]-1]+=1;
					max10=1;
				}else{
					max10=0;
				}
			}
			if(max10) cout<<"1";
			cout<<in[NMin]<<endl;
		}
	}
	return 0;
}

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

#define debug(x) cout<<#x<<": "<<x<<endl
#define rep(i,n) for (int i=0;i<(n);i++)
#define FOR(i,a,b) for (int i=(a);i<=(b);i++)
#define all(a) (a).begin(),(a).end()
using namespace std;
typedef vector<int> VI;
typedef vector<vector<int>> VVI;
typedef long long ll;

void solve() {
	int n;
	cin >> n;
	rep(i, n) {
		vector<ll> v1(11), v2(11);
		string s1, s2;
		cin >> s1 >> s2;
		rep(j, 10) {
			if (s1.size() > 10 * j) v1[j] = stoi(s1.substr(10 * j, (s1.size() - 10 * j >= 10 ? 10 : s1.size() - 10 * j)));
			if (s2.size() > 10 * j) v2[j] = stoi(s2.substr(10 * j, (s2.size() - 10 * j >= 10 ? 10 : s2.size() - 10 * j)));
		}
		rep(j, 11) {
			v1[j] += v2[j];
			if (v1[j] >= 1e10) {
				v1[j] -= 1e10;
				v2[j]++;
			}
		}
		if (v1[8] || v1[9] || v1[10]) cout << "overflow" << endl;
		else {
			rep(j, 11) if (v1[10 - j] != 0) cout << v1[10 - j];
			cout << endl;
		}
	}

}




int main() {
	cin.tie(0);
	ios::sync_with_stdio(false);


	solve();
	//system("PAUSE");
	return 0;
}

C++: #include <iostream>
#define MAX 80

#include <string>

using namespace std;

int main()
{
	int n;
	cin >> n;
	for (int counter = 0; counter < n; counter++){
		string a, b;
		cin >> a >> b;
		if (a.length() > MAX || b.length() > MAX){
			cout << "overflow" << endl;
			continue;
		}
		int inta[MAX + 2], intb[MAX + 2], sum[MAX + 2];
		fill(inta, inta + MAX + 2, 0);
		fill(intb, intb + MAX + 2, 0);
		fill(sum, sum + MAX + 2, 0);
		for (int i = 0; i < (signed)a.length(); i++){
			inta[i] = a[a.length() - i - 1] - '0';
		}
		for (int i = 0; i < (signed)b.length(); i++){
			intb[i] = b[b.length() - i - 1] - '0';
		}
		
		int over = 0;
		for (int i = 0; i < MAX + 2; i++){
			sum[i] = inta[i] + intb[i] + over;
			if (sum[i] >= 10){
				sum[i] -= 10;
				over = 1;
			}
			else{
				over = 0;
			}
		}

		for (int i = MAX + 1;; i--){
			if (sum[i] != 0){
				if (i >= MAX){
					cout << "overflow" << endl;
					break;
				}
				for (int j = i; j >= 0; j--){
					cout << sum[j];
				}
				cout << endl;
				break;
			}
		}
	}
	return 0;
}