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,x1,x2,y1,y2,p,q,a=0,b=0;
	vector<int> E,F;
	vector<long long int> S, T;
	vector<vector<long long int>> I,J;
	string s, t;
	vector<string> U;
	vector<vector<string>> UU;

	cin >> n;
	for (int k = 0; k < n; k++) {
		cin >> s >> t;
		int f = 0;
		if (s.size() > 80 || t.size() > 80) {
			f = 1;
			F.push_back(f);
		}
		else F.push_back(0);
		x1 = s.size() % 10; y1 = (s.size()+9) / 10;p = 8 - y1;
		for (int i = 0; i < p; i++)S.push_back(0);
		if (x1 == 0) for (int i = 0; i < y1; i++)S.push_back(stoll(s.substr(i * 10, 10)));
		else {
			S.push_back(stoll(s.substr(0, x1)));
			for (int i = 1; i < y1; i++)S.push_back(stoll(s.substr(x1 + i * 10, 10)));
		}

		x2 = t.size() % 10; y2 = (t.size()+9) / 10;q = 8 - y2;
		for (int i = 0; i < q; i++)T.push_back(0);
		if (x2 == 0) for (int i = 0; i < y2; i++)T.push_back(stoll(t.substr(i * 10, 10)));
		else {
			T.push_back(stoll(t.substr(0, x2)));
			for (int i = 1; i < y2; i++)T.push_back(stoll(t.substr(x2 + i * 10, 10)));
		}
		I.push_back(S); S.clear();
		J.push_back(T); T.clear();
	}
	for (int k = 0; k < I.size(); k++) {
		for (int i = 0; i < 8; i++) U.push_back(to_string(I[k][i] + J[k][i]));
		UU.push_back(U); U.clear();
	}
	
	for (int k = 0; k < I.size(); k++) {
		for (int i = 7; i >= 0; i--) {
			UU[k][i] = to_string(stoll(UU[k][i]) + b);
			if (UU[k][i].size() == 11 && i == 0) {
				UU[k][i] = "a";
				b = 1;
			}
			else if (UU[k][i].size() == 11) {
				UU[k][i] = UU[k][i].substr(1, 10);
				b = 1;
			}
			else if (UU[k][i].size() < 10) {
				for (int j = 0; j < 10 - UU[k][i].size()+j; j++) UU[k][i] = "0" + UU[k][i];
				b = 0;
			}
			else b = 0;
		}
		b = 0;
	}

	for (int k = 0; k < UU.size(); k++) {
		int e = 0;
		for (int i = 0; i < UU[k].size(); i++) {
			if (UU[k][i] == "a") {
				e = 100;
				break;
			}
			else if (stoll(UU[k][i]) == 0);
			else {
				e = i;
				E.push_back(e);
				break;
			}
		}
	}

		for(int k=0;k<UU.size();k++){
			if (E[k] == 100)cout << "overflow" << endl;
			else if (F[k] == 1)cout << "overflow" << endl;
			else {
				cout << stoll(UU[k][E[k]]);
				for (int i = E[k]+1; i < UU[k].size(); i++) cout << UU[k][i];
			}
			cout << endl;
		}
	return 0;
}

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

using namespace std;

const int N = 85;

char a[N], b[N];

void reverse(char a[]) {
	int len = strlen(a);
	for (int i=0, j=len-1; i<j; ++i, --j) {
		char tmp = a[i];
		a[i] = a[j];
		a[j] = tmp;
	}
}

void add(char a[], char b[]) {
	int lenA = strlen(a);
	int lenB = strlen(b);
	int i, j, tmp, jin = 0;
	for (i=0; i<lenA && i<lenB; ++i) {
		tmp = (a[i] - '0') + (b[i] - '0') + jin;
		a[i] = tmp % 10 + '0';
		jin = tmp / 10;  
	}
	if (!a[i] && !b[i] && jin) {
		a[lenA] = jin + '0';
		a[lenA + 1] = '\0';
	}
	else if (a[i] && !b[i] && jin) {
		for (; i<lenA; ++i) {
			tmp = a[i] - '0' + jin;
			a[i] = tmp % 10 + '0';
			jin = tmp / 10;
		}
		if (jin) {
			a[lenA] = jin + '0';
			a[lenA + 1] = '\0';
		} 
	}
	else if (!a[i] && b[i] && jin) {
		for (; i<lenB; ++i) {
			tmp = b[i] - '0' + jin;
			a[i] = tmp % 10 + '0';
			jin = tmp / 10;
		}
		if (jin) {
			a[lenB] = jin + '0';
			a[lenB + 1] = '\0';
		}
		else a[lenB] = '\0';
	}
}

int main() {
	int t;
	scanf ("%d", &t);
	while (t--) {
		scanf ("%s%s", a, b);
		if (strlen(a) > 80 || strlen(b) > 80) printf ("overflow\n");
		else {
			reverse(a);
			reverse(b);
			add(a, b);
			if (strlen(a) > 80) printf ("overflow\n");
			else reverse(a), printf ("%s\n", a);
		}
	}
}

C++: #include <iostream>
#include <algorithm>
#include <cctype>
#include <cstdio>
#include <math.h>
#include <map>
#include <queue>
#include <string>
using namespace std;

int N,ans[82],a,b,la,lb,up,deg;
string sta,stb;

int main(){
	cin>>N;
	for(int i=0;i<N;i++){
		cin>>sta>>stb;
		la=sta.size();
		lb=stb.size();
		up=0;
		deg=1;
		for(int i=0;i<max(la,lb)+1;i++){
			if(i>=la)a='0';
			else a=sta[la-1-i];
			if(i>=lb)b='0';
			else b=stb[lb-1-i];
			if(a+b+up>105){
				ans[i]=a+b+up-10-96;
				up=1;
			}
			else{
				ans[i]=a+b+up-96;
				up=0;
			}
			if(ans[i]!=0)deg=i+1;
		}
		if(deg>80)cout<<"overflow"<<endl;
		else{
			for(int i=0;i<deg;i++)cout<<ans[deg-1-i];
			cout<<endl;
		}
	}
}

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

using namespace std;
int main(){
	int n,l,m,c;
	char a[90],b[90],s[90];
	cin >> n;
	for(int i=0;i<n;i++){
		for(int j=0;j<90;j++)s[j]=0;
		scanf("%s%s",a,b);
		l = strlen(a);
		m = strlen(b);
		reverse(a,a+l);
		reverse(b,b+m);
		c=0;
		for(int j=0;j<max(l,m);j++){
			int o=c;
			if(j<l)o += a[j]-'0';
			if(j<m)o += b[j]-'0';
			c = o>9;
			s[j] = o%10 + '0';
		}
		l=max(l,m);
		if(c>0)s[l++]='1';
		reverse(s,s+l);
		if(l>80)puts("overflow");
		else puts(s);
	}
	return 0;
}

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

int main()
{
	int n;
	cin >> n;
	for (int counter = 0; counter < n; counter++){
		string a, b;
		cin >> a >> b;
		int inta[82], intb[82], sum[82];
		fill(inta, inta + 82, 0);
		fill(intb, intb + 82, 0);
		fill(sum, sum + 82, 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 < 82; i++){
			sum[i] = inta[i] + intb[i] + over;
			if (sum[i] >= 10){
				sum[i] -= 10;
				over = 1;
			}
			else{
				over = 0;
			}
		}

		for (int i = 81;; i--){
			if (sum[i] != 0){
				for (int j = i; j >= 0; j--){
					cout << sum[j];
				}
				cout << endl;
				break;
			}
		}
	}
	return 0;
}