Go: package main

import (
	"bufio"
	"fmt"
	"math/big"
	"os"
	"strconv"
)

var scanner = bufio.NewScanner(os.Stdin)

func nextString() string {
	scanner.Scan()
	return scanner.Text()
}

func nextInt() int {
	n, err := strconv.Atoi(nextString())
	if err != nil {
		fmt.Printf("strconv.Atoi failed: %v\n", err)
	}
	return n
}

func main() {
	scanner.Split(bufio.ScanWords)

	const maxCapacity = 512 * 1024
	buf := make([]byte, maxCapacity)
	scanner.Buffer(buf, maxCapacity)

	n := nextInt()
	maxint, _ := new(big.Int).SetString("100000000000000000000000000000000000000000000000000000000000000000000000000000000", 10)

	for i := 0; i < n; i++ {
		aStr := nextString()
		bStr := nextString()

		a, _ := new(big.Int).SetString(aStr, 10)
		b, _ := new(big.Int).SetString(bStr, 10)
		c := big.NewInt(0).Add(a, b)

		if c.Cmp(maxint) < 0 {
			fmt.Println(c)
		} else {
			fmt.Println("overflow")
		}
	}
}

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-1) * 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;
		}
		getchar();
		getchar();
	return 0;
}

C++: #include <bits/stdc++.h>
using namespace std ;

#define pb(n) push_back(n)
#define fi first
#define se second
#define np string::npos
#define X real()
#define Y imag()
#define value(x,y,w,h) (x >= 0 && x < w && y >= 0 && y < h)
#define all(r) (r).begin(),(r).end()
#define gsort(st,en) sort((st),(en),greater<int>())
#define vmax(ary) *max_element(all(ary))
#define vmin(ary) *min_element(all(ary))
#define debug(x) cout<<#x<<": "<<x<<endl
#define fcout(n) cout<<fixed<<setprecision((n))
#define scout(n) cout<<setw(n)

#define rep(i,n) for(int i = 0; i < (int)(n);++i)
#define repc(i,a,b) for(int i = (a);i < (int)(b);++i)
#define repi(it,array) for(auto it = array.begin(),end = array.end(); it != end;++it)
#define repa(n,array) for(auto &n :(array))

typedef long long ll ;
typedef deque<int> di ;
typedef deque<ll> dl ;
typedef map<string,int> dict;
typedef complex<double> comd;
typedef pair<int,int> pii;

constexpr int imax = ((1<<30)-1)*2+1 ;
constexpr int inf = 100000000;
constexpr double PI = acos(-1.0) ;
double eps = 1e-10 ;
const int dy[] = {-1,0,1,0};
const int dx[] = {0,-1,0,1};

double CalcDist(comd p1, comd p2){
  return sqrt(pow(p1.X - p2.X,2.0) + pow(p1.Y -p2.Y,2.0));
}

  template <typename T>
void out(deque < T > d)
{
  for(size_t i = 0; i < d.size(); i++)
  {
    debug(d[i]);
  }
}

template<typename T>
T ston(string& str, T n){
  istringstream sin(str) ;
  T num ;
  sin >> num ;
  return num ;
}

unsigned long long int ulli;

int main(){
  cin.tie(0);
  ios::sync_with_stdio(false);
  int N;
  string s1,s2,ans(105,' ');
  cin >> N;
  rep(i,N){
    cin >> s1 >> s2;
    ans = string(105,' ') ;
    if(s1.size()> 80 || s2.size() > 80){
      cout << "overflow" << endl;
      continue;
    }
    int a,b,c = 0;
    bool f = s1.size() >= s2.size();
    for(int i = 0;i <min(s1.size(),s2.size());++i){
      if(f){
        a = s1[s1.size()-1-i] - '0' ;
        b = s2[s2.size()-1-i] - '0' ;
      }
      else{
        a = s1[s1.size()-1-i] - '0' ;
        b = s2[s2.size()-1-i] - '0' ;
      }
//       debug((a+b+c)%10+'0');
      ans[ans.size()-1-i] = (a+b+c)%10+'0';
      c = (a+b+c)/10;
//       cout << ans.size()-1-i << " :" << a << " :" << b << " :" << c <<endl;
    }
    for(int i = min(s1.size(),s2.size()); i < max(s1.size(),s2.size());++i){
      if(f){
        ans[ans.size()-1-i] = (s1[s1.size()-1-i] - '0' + c)%10+'0';
        c = (s1[s1.size()-1-i] - '0' + c )/10;
      }
      else{
        ans[ans.size()-1-i] = (s2[s2.size()-1-i] + c - '0')%10+'0';
        c = (s2[s2.size()-1-i] + c - '0')/10;
      }
    }
    if(c > 0){
//       cout << ans << "c :"<<  c << endl;
      ans[0] = c +'0' ;
    }
    if(ans.size()-count(all(ans),' ') > 80){
      cout << "overflow" << endl;
      continue;
    }
    for(size_t c = ans.find_first_of(" "); c != string::npos; c = ans.find_first_of(" ")){
      ans.erase(c,1);
    }
    cout << ans << endl;
  }
  return 0;
}

C++: #include <iostream>
#include <string>
#include <utility>
using namespace std;
int toNumber(char c){
	switch (c)
	{
	case '0': return 0;
	case '1': return 1;
	case '2': return 2;
	case '3': return 3;
	case '4': return 4;
	case '5': return 5;
	case '6': return 6;
	case '7': return 7;
	case '8': return 8;
	case '9': return 9;
	}
	return 0;
}
int main()
{
	int n, sa, sb, k, r, i;
	string a, b;
	int sum[81] = {};
	cin >> n;
	while (n--){
		cin >> a >> b;
		if (a.size() < b.size()) swap(a, b);
		sa = a.size();
		sb = b.size();
		k = i = 0;
		for (; i < sa; ++i){
			if (i < sb) k = toNumber(a[sa - 1 - i]) + toNumber(b[sb - 1 - i]) + k;
			else 		k = toNumber(a[sa - 1 - i]) + k;
			r = k % 10;
			k = k / 10;
			sum[i] = r;
		}
		if (0 != k) sum[i] = k;
		else --i;
		for (int d = i; d >= 0; --d)
			cout << sum[d];
		cout << "\n";
	}
	return 0;
}

C++: #include<algorithm>
#include<iostream>
#include<cstdio>
#include<stdio.h>
#include<string>
#include<string.h>
#include <math.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 = (long long)log10(a) + 1;;
		a2 = (long long)log10(b) + 1;;
		if (a1 + a2 >= 80) { printf("overflow"); }
		else { c = a + b; printf(c + "\n"); }
	}
	return 0;
}