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 <fstream>

using namespace std;

#define MAX_INT_LENGTH 80

//large integer upto 80 digits
struct largeInt{
    int vals[MAX_INT_LENGTH]; // MAX_INT_LENGTH digits
};

void toLargeInt(string str, largeInt &lv){
    int start = MAX_INT_LENGTH-str.length();
    for (int i = 0; i < MAX_INT_LENGTH; i++){
        if (i >= start)
            lv.vals[i] = (str[i-start] - '0');
        else
            lv.vals[i] = 0;
    }
}

largeInt addLargeInt(const largeInt &lv1, const largeInt &lv2){
    largeInt sum;
    int carry = 0;
    for (int i = MAX_INT_LENGTH-1; i >= 0; i--){
        int sub_sum = lv1.vals[i] + lv2.vals[i] + carry;
        sum.vals[i] = sub_sum % 10;
        carry = sub_sum / 10;
    }
    return sum;
}

string toString(const largeInt &v){
    string str(MAX_INT_LENGTH,'0');
    for (int i = 0; i < MAX_INT_LENGTH; i++)
        str[i] = v.vals[i] + '0';
    return str;
}

int main()
{
    //ifstream cin("input.txt");
    
    // input data
    int n;
    cin >> n;
    for (int i = 0; i < n; i++){
        string v1,v2;
        cin >> v1 >> v2;
        largeInt lv1, lv2;
        if (lv1.length()>MAX_INT_LENGTH || lv2.length()>MAX_INT_LENGTH){
            cout << "overflow" << endl;
            continue;
        }
        toLargeInt(v1,lv1);
        toLargeInt(v2,lv2);
        largeInt sum = addLargeInt(lv1,lv2);
        cout << toString(sum) << endl;
    }
   return 0;
}

C++: #include<iostream>
#include<stdio.h>
#include <algorithm>
#include<string.h>
#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 alen,blen,mlen;
	cin>>m;
	while(m--){
		for(int i=0;i<90;i++){
			a[i]=b[i]=ans[i]=0;
		}
		cin>>in;
		cin>>in2;
		alen=in.size();
		blen=in2.size();
		for(int i=0;i<alen;i++){
			a[alen-1-i]=in[i]-'0';
		}
		for(int i=0;i<blen;i++){
			b[blen-1-i]=in[i]-'0';
		}
		mlen=max(alen,blen);
		if(mlen>80){
			cout<<"overflow"<<endl;
			continue;
		}
		f=0;
		for(int i=0;i<mlen+1;i++){
			ans[i]=(a[i]+b[i]+f)%10;
			f=(a[i]+b[i]+f)/10;
		}
		if(ans[mlen]!=0){
			if(mlen>79){
				//cout<<"overflow"<<endl;
				continue;
			}
			//cout<<ans[mlen];
		}
		
		for(int i=0;i<mlen;i++){
			//cout<<ans[mlen-1-i];
		}cout<<endl;
		
		
	}
	return 0;
}

C++: // 2012/10/28 Tazoe

#include <iostream>
#include <string>
using namespace std;

int main()
{
	int N;
	cin >> N;

	for(int i=0; i<N; i++){
		string S1, S2;
		cin >> S1;
		cin >> S2;

		if(S1.size()<S2.size()){
			string tmp = S1;
			S1 = S2;
			S2 = tmp;
		}

		int A[80];
		int C = 0;
		for(int j=0; j<S1.size(); j++){
			int tmp = (int)(S1[S1.size()-1-j]-'0')+C;
			if(j<S2.size()){
				tmp += (int)(S2[S2.size()-1-j]-'0');
			}

			A[j] = tmp%10;
			C = tmp/10;
		}

		if(S1.size()==80&&C==1){
			cout << "overflow" << endl;
		}
		else{
			if(C==1){
				cout << C;
			}
			for(int j=S1.size()-1; j>=0; j--){
				cout << A[j];
			}
			cout << endl;
		}
	}

	return 0;
}

C++: #include<iostream>
using namespace std;
int main(){
  int n;
  cin >>n;
  while(n--){
    string a,b,solve = "";
    cin >>a>>b;
    int as = a.size(),bs = b.size(),ms;
    int j=0,k;
    ms = (as<bs)?bs:as;
    for(int i=0; i<ms; i++){
      if(i<as && i<bs) k = a[as-i-1]+b[bs-i-1]-'0'*2+j;
      else if(i<as) k = a[as-i-1]-'0'+j;
      else if(i<bs) k = b[bs-i-1]-'0'+j;
      j = k/10;
      solve+='0'+k%10;
    }
    if(j!=0) solve+='1';
    if(solve.size()>80) cout <<"overflow"<<endl;
    else{
      for(int i=solve.size()-1; i>=0; i--)
	cout <<solve[i];
      cout <<endl;
    }
  }
  return 0;
}

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

int main()
{
    int N,j;
    cin >> N;
    for(int i=0 ; i<N ; i++){
        int ans[81]={0};
        ans[0]=11111;
        char strA[100],strB[100];
        cin >> strA >> strB;
        if(strlen(strA)>80||strlen(strB)>80){
            cout << "overflow" << endl;
            continue;
        }
        char str1[strlen(strA)][2],str2[strlen(strB)][2];
        for(int i=0 ; i<strlen(strA) ; i++){
            str1[i][0]=strA[i];
            str1[i][1]='\0';
        }
        for(int i=0 ; i<strlen(strB) ; i++){
            str2[i][0]=strB[i];
            str2[i][1]='\0';
        }
        int a[81]={0},b[81]={0};
        j=strlen(strA)-1;
        for(int i=80 ; j>=0 ; i--){
            a[i]=atoi(str1[j]);
            j--;
        }
        j=strlen(strB)-1;
        for(int i=80 ; j>=0 ; i--){
            b[i]=atoi(str2[j]);
            j--;
        }
        for(int i=80 ; i!=0 ; i--){
            ans[i]=ans[i]+a[i]+b[i];
            if(ans[i]>=10){
                ans[i]=ans[i]-10;
                ans[i-1]++;
            }
        }
        if(ans[0]!=11111){
            cout << "overflow" << endl;
            continue;
        }
        for(int i=1 ; i<81 ; i++){
            if(ans[i]!=0){
                j=i;
                break;
            }
        }
        char c[2];
        for(int i=j ; i<81 ; i++){
            itoa(ans[i],c,10);
            cout << c;
        }
        cout << '\n';
    }
    return 0;
}

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

int main() {
	int n;
	cin >> n;
	for(int k = 0; k < n; k++) {
		string str1, str2;
		cin >> str1 >> str2;
		int a = str1.size(), b = str2.size();
		if(a > 80 || b > 80) {
			cout << "overflow" <<endl;
		} else if(a >= b) {
			for(int i = 0; i < b; i++) {
				str1[a - b + i] += str2[i] - '0';
				for(int i = a - 1; i >= 0; i--) {
					if(str1[i] > '9') {                 //?????????//
						if(i != 0) {
							str1[i] = str1[i] - ':' + '0';
							str1[i - 1] = str1[i - 1] + 1;
						}
					}
				}
			}
			if(a == 80 && str1[0] > '9') {
				cout << "overflow" <<endl;
			} else {
				for (int j = 0; j < a; j++) {
					if(str1[j] > '9') {
						printf("1%c",str1[j] - ':' + '0');
					} else cout<< str1[j];
				}
			}
		} else if(b > a) {
			for(int i = 0; i < a; i++) {
				str2[b - a + i] += str1[i] - '0';
				for(int i = b - 1; i >= 0; i--) {
					if(str2[i] > '9') {                    //?????????//
						if(i != 0) {
							str2[i] = str2[i] - ':';
							str2[i - 1] = str2[i - 1] + 1;
						}
					}
				}
			}
			if(b == 80 && str2[0] > '9') {
					cout << "overflow" <<endl;
			} else {
				for (int j = 0; j < b; j++) { 
					if(str2[j] > '9') {
						printf("1%c",str2[j] - ':' + '0');
					} else cout << str2[j];
				}
			}
		}	
		cout <<endl;
	}
	return 0;
}