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 <algorithm>

using namespace std;

void tasu(string s1, string s2){
	int num[2][max(s1.length(), s2.length())+1];
	for(int i = 0; i < sizeof(num[0])/sizeof(int); i++){
		if(s1.length() > i){
			num[0][i] = (int)s1.at(s1.length()-1-i) - (int)'0';
		}else{
			num[0][i] = 0;
		}
		if(s2.length() > i){
			num[1][i] = (int)s2.at(s2.length()-1-i) - (int)'0';
		}else{
			num[1][i] = 0;
		}
	}
	for(int i = 0; i < sizeof(num[0])/sizeof(int)-1; i++){
		num[1][i] += num[0][i];
		if(num[1][i] > 9){
			num[1][i+1] += (int)(num[1][i]/10);
			num[1][i] %= 10;
		}
	}
	int keta = sizeof(num[0])/sizeof(int);
	while(keta && !num[1][keta-1]){
		keta--;
	} 
	if(keta > 80){
		cout << "overflow" << endl;
	}else if(keta){
		while(keta--){
			cout << num[1][keta];
		}
		cout << endl;
	}else{
		cout << 0 << endl;
	}
}
int main(){
	int n;
	cin >> n;
	while(n--){
		string s1, s2;
		cin >> s1 >> s2;
		tasu(s1, s2);
	}
}

C++: #include<iostream>
#include<stdio.h>
#include <algorithm>
#include<string>
#include <utility>
using namespace std;
using namespace std;
int main(){
	int m;
	string in,in2;
	int a[90],b[90],ans[90];
	int f;
	int a_len,b_len,m_len;
	cin>>m;
	while(m--){
		for(int i=0;i<90;i++){
			a[i]=b[i]=ans[i]=0;
		}
		cin>>in;
		cin>>in2;
		a_len=in.size();
		b_len=in2.size();
		for(int i=0;i<a_len;i++){
			a[a_len-1-i]=in[i]-'0';
		}
		for(int i=0;i<b_len;i++){
			b[b_len-1-i]=in2[i]-'0';
		}
		m_len=max(a_len,b_len);
		if(m_len>80){
			cout<<"overflow"<<endl;
			continue;
		}
		f=0;
		for(int i=0;i<m_len+1;i++){
			ans[i]=(a[i]+b[i]+f)%10;
			f=(a[i]+b[i]+f)/10;
		}
		if(ans[m_len]!=0){
		}
		
		
	}
	
	
	return 0;
}

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

int main(){
	int n,i,j,flag;
	char ch1[100],ch2[100];
	cin >> n;
	fgets(ch1,100,stdin);
	for(i = 0;i < n;i++){
		fgets(ch1,100,stdin);
		fgets(ch2,100,stdin);
		if(strlen(ch1) > 80 || strlen(ch2) > 80){
			cout << "overflow" << endl;
		}
		else {
			stack<int> st1,st2,sum;
			sum.push(0);
			for(j = 0;j < strlen(ch1) - 1;j++) st1.push(ch1[j] - '0');
			for(j = 0;j < strlen(ch2) - 1;j++) st2.push(ch2[j] - '0');
			while(!st1.empty() || !st2.empty()){
				if(st1.empty()) st1.push(0);
				if(st2.empty()) st2.push(0);
				flag = sum.top();
				sum.pop();
				sum.push((st1.top() + st2.top() + flag) % 10);
				sum.push((st1.top() + st2.top() + flag) / 10);
				st1.pop();
				st2.pop();
			}
			if(!sum.top()) sum.pop();
			if(sum.size() > 80) cout << "overflow" << endl;
			for(;!sum.empty();) {
				cout << sum.top();
				sum.pop();
			}
			cout << endl;
		}
	}
	return 0;
}

C++: #include <iostream>
#include <cstdio>
#include <cstring>
#include <algorithm>
using namespace std;
int main(){
  int n, maxlen;
  char a[10001], b[10001];

  cin >> n;
  for(int ii = 0 ; ii < n ; ii++){
    string ans = "";
    scanf("%s%s", a, b);
    
    maxlen = max(strlen(a), strlen(b));

    reverse(a, a+strlen(a));
    reverse(b, b+strlen(b));
    
    for(int i = strlen(a) ; i < 10000 ; i++){
      a[i] = '0';
    }
    for(int i = strlen(b) ; i < 10000 ; i++){
      b[i] = '0';
    }

    int tmp, tmp2 = 0;
    bool flag = 0;

    for(int i = 0 ; i < maxlen ; i++){
      tmp = (a[i] - '0') + (b[i] - '0') + tmp2;
      
      if(i == maxlen - 1){
	if(tmp >= 10){
	  tmp2 = 1;
	  tmp -= 10;
	  ans += tmp + '0';
	  ans += '1';
	}
	else ans += tmp + '0';
      }

      else if(tmp >= 10){
	tmp2 = 1;
	tmp -= 10;
	ans += tmp + '0';
      }
      else if(tmp <= 9){
	ans += tmp + '0';
	tmp2 = 0;
      }
    }

    reverse(ans.begin(), ans.end());

    int size = ans.size();

    if(size >= 80 || maxlen >= 80) cout << "overflow" << endl;
    else cout << ans << endl;
  }
  return 0;
}

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

int main(){
	int i,j,k,n,tmp,flag,length;
	int yosan[20];
	char str[81],str2[256];

	scanf("%d",&n);

	for(i = 0;i < n;i++){
		memset(yosan,0,sizeof(yosan));
		flag = 0;
		for(j = 0;j < 2;j++){
			scanf("%s",str2);
			length = strlen(str2);
			if(flag && length >= 81){
				flag = 1;
				break;
			}
			memset(str,'\0',sizeof(str));
			for(k = 0;k < 80;k++){
				if(k >= 80-length){
					str[k] = str2[k-80+length];
				}else{
					str[k] = '0';
				}
			}
			//printf("%s\n",str);
			tmp = 0;
			for(k = 0;k < 80;k++){
				tmp *= 10;
				tmp += (str[k] - '0');
				if(k % 4 == 3){
					yosan[k/4] += tmp;
					tmp = 0;
				}
			}
			for(k = 29;k > 0;k--){
				if(yosan[k] >= 10000){
					yosan[k-1]++;
					yosan[k] %= 10000;
				}
			}
			if(yosan[0] >= 10000){
				flag = 1;
				break;
			}
		}
		if(!flag){
			tmp = 0;
			while(yosan[tmp] == 0){
				tmp++;
			}
			printf("%d",yosan[tmp]);
			tmp++;
			for(j = tmp;j < 20;j++){
				printf("%04d",yosan[j]);
			}
			printf("\n");
		}else{
			printf("overflow\n");
		}
	}

	return 0;
}

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

#define times(n, i) uptil(0, n, i)
#define upto(f, t, i) for(auto _##i = (t), i = decltype(_##i)(f) poi  i <= _##i poi  i++)
#define uptil(f, t, i) for(auto _##i = (t), i = decltype(_##i)(f) poi  i < _##i poi  i++)
#define downto(f, t, i) for(auto i = (f), _##i = decltype(i)(t) poi  i >= _##i poi  i--)
#define downtil(f, t, i) for(auto i = (f), _##i = decltype(i)(t) poi  i > _##i poi  i--)

#define unless(c) if(!(c))
#define until(c) while(!(c))
#define loop while(true)

#define poi ;
#define poipoi << "\n"

#define fuwafuwa(n, c) dokidoki((n) + (c) - 1, c)
#define dokidoki(n, c) ((n) / (c) * (c))

template<class T> bool read(T& t){
    return cin >> t poi
}
template<class T, class...U> bool read(T& t, U&... u){
    unless(cin >> t) return false poi
    return read(u...) poi
}

const int PYON = 80 poi

int main(){
    int N poi
    cin >> N poi
    times(N, i){
        string A, B poi
        cin >> A >> B poi
        if(A.size() > PYON || B.size() > PYON){
            cout << "overflow" poipoi poi continue poi
        }
        vector<int> a(PYON, 0), b(PYON, 0) poi
        upto(1, A.size(), i) a[PYON - i] = A[A.size() - i] - '0' poi
        upto(1, B.size(), i) b[PYON - i] = B[B.size() - i] - '0' poi
        int pyon = 0 poi
        vector<int> sum(PYON) poi
        downto(PYON - 1, 0, i){
            sum[i] = a[i] + b[i] + pyon poi
            pyon = sum[i] / 10 poi
            sum[i] %= 10 poi
        }
        if(pyon) cout << "overflow" poi
        else times(PYON, i) if((pyon = pyon || sum[i])) cout << sum[i] poi
        cout poipoi poi
    }
    return 0 poi
}