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<stdio.h>
#include<string>
#include<vector>
#include<string.h>
#include<stdlib.h>
#include<math.h>

using namespace std;

int main(){
	int n;
	string ret1,ret2;
	vector<int> result;
	int num = 0, ret = 0;
	int j;

	cin >> n;

	for (int i = 0;i < n;i++){

		ret = 0;

		cin >> ret1 >> ret2;

		if (ret1.size() > 80 || ret2.size() > 80){
			cout << "overflow" << endl;
		}

		for (j = 1;j <= ret1.size() && j <= ret2.size();j++){
			num = (ret1.at(ret1.size() - j) - '0' + ret2.at(ret2.size() - j) - '0' + ret) % 10;
			ret = (ret1.at(ret1.size() - j) - '0' + ret2.at(ret2.size() - j) - '0' + ret);
			ret = (ret - num)/10;
			result.push_back(num);
		}

		if (ret1.size() == ret2.size()){
			if (ret != 0)
				result.push_back(ret);
		}
		else if(ret1.size() > ret2.size()){
			for (; j <= ret1.size(); j++){
				num = (ret1.at(ret1.size() - j) - '0' + ret) % 10;
				ret = ret1.at(ret1.size() - j) - '0' + ret;
				ret = (ret - num) / 10;
				result.push_back(num);
			}
			if (ret != 0)
				result.push_back(ret);
		}
		else if (ret1.size() < ret2.size()){
			for (; j <= ret2.size(); j++){
				num = (ret2.at(ret2.size() - j) - '0' + ret) % 10;
				ret = ret2.at(ret2.size() - j) - '0' + ret;
				ret = (ret - num) / 10;
				result.push_back(num);
			}
			if (ret != 0)
				result.push_back(ret);
		}


		while (result.empty() != true){
			cout << result[result.size()-1];
			result.pop_back();
		}
		cout << endl;

	}


	return 0;
}

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

int main(){
  int j;
  int n;
  char s1[111];
  char tmp[111];
  char s2[111];
  char ans[180];
  int  kuri = 0;
  cin >> n;
  while(n--){
  memset(ans,0,sizeof(ans));
  memset(s1,0,sizeof(s1));
  memset(s2,0,sizeof(s2));
  memset(tmp,0,sizeof(tmp));
  cin >> s1 >> s2;
  if(strlen(s1) < strlen(s2)){
      for(int i=0,j=strlen(s1)-1;i<=j;i++,j--) swap(s1[i],s1[j]);
      for(int i=0,j=strlen(s2)-1;i<=j;i++,j--) swap(s2[i],s2[j]);
      for(int i=0;i<strlen(s2);i++){
	if((s2[i] + (s1[i]%'0') + kuri) > '9'){
	  ans[i] = (s2[i] + (s1[i]%'0') + kuri)-10;
	  kuri = 1;
	}
	else {
	  ans[i] = (s2[i] + (s1[i]%'0') + kuri);
	  kuri = 0;
	}
	if(i == strlen(s2)-1 && kuri == 1) ans[i+1] = '1';
      }
      if(strlen(ans) > 80) cout << "overflow" << endl;
      else{
	for(int i=strlen(ans)-1;i>=0;i--) cout << ans[i];
      }
  }
  else{
    for(int i=0,j=strlen(s1)-1;i<=j;i++,j--) swap(s1[i],s1[j]);
    for(int i=0,j=strlen(s2)-1;i<=j;i++,j--) swap(s2[i],s2[j]);
    for(int i=0;i<strlen(s1);i++){
      if((s1[i] + (s2[i]%'0') + kuri) > '9'){
	ans[i] = (s1[i] + (s2[i]%'0') + kuri)-10;
	kuri = 1;
      }
      else {
	ans[i] = (s1[i] + (s2[i]%'0') + kuri);
	kuri = 0;
      }
      if(i == strlen(s1)-1 && kuri == 1) ans[i+1] = '1';
    }
    if(strlen(ans) > 80) cout << "overflow" << endl;
    else{
      for(int i=strlen(ans)-1;i>=0;i--) cout << ans[i];
    }
  }
  cout << endl;
  }
}

C++: #include<iostream>
#include<string.h>
#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[81],strB[81];
        cin >> strA >> strB;
        if(strlen(strA)>80||strlen(strB)>80){
            cout << "overflow" << endl;
            break;
        }
        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};
        a[0]=11111;
        b[0]=11111;
        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;
            break;
        }
        for(int i=1 ; i<81 ; i++){
            if(ans[i]!=0){
                j=i;
                break;
            }
        }
        for(int i=j ; i<81 ; i++){
         cout << ans[i];
        }
        cout << '\n';
    }
    return 0;
}

C++: #include<string>
#include<iostream>
#include<algorithm>
#include<cstring>

using namespace std;
string a,b,sum;
void check(int z,int x);
int A,B;
int main(void){

    int n;
    cin>>n;
    while(n--){
        a="",b="",sum="";

        cin>>a>>b;
        //cout<<"a="<<a<<"b="<<b<<"sum="<<sum<<endl;
        if(a.length()>80||b.length()>80){
            sum="overflow";
            goto end;
        }
        A=a.length(),B=b.length();
        //cout<<A<<B<<endl;
        reverse(a.begin(),a.end());
        reverse(b.begin(),b.end());
        if(A<B)swap(A,B),swap(a,b);
        for(int i=0;i<=A-B;i++){
            b+="0";
        }
        check(0,0);
        //cout<<sum<<endl;
        if(sum.length()>80){
            sum="overflow";
            goto end;
        }
        reverse(sum.begin(),sum.end());
        end:;
        cout<<sum<<endl;
    }
    return 0;
}

void check(int z,int x){
    int i;


    if(a[z]==0)a[z]='0';
    if(b[z]==0)b[z]='0';
    //cout<<a[z]<<"."<<b[z]<<"."<<sum<<endl;
    //cout<<"--"<<z<<endl;
    if(z==80||(z>A&&z>B&&x==1)||(z>=A&&z>=B&&x==0)){
            x=0;
            return;
    }
    i=(int)a[z]-'0' + (int)b[z]-'0' + x,x=0;
    //cout<<"*"<<i<<endl;
    //cout<<"-"<<z<<endl;
    if(i>=10)x=1,i-=10;
    sum+=i+'0';
    //cout<<"+"<<sum<<endl;

    check(z+1,x);

}

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

struct Integer{
    int num[100];
    Integer(const char *s){
        fill_n(num,100,0);
        int l=strlen(s);
        for(int i=0;i<l;i++){
            num[l-i-1]=s[i]-'0';
        }
    }
};
const Integer operator+(const Integer &lint,const Integer &rint){
    Integer ret("");
    for(int i=0;i<100;i++){
        int v=ret.num[i]+lint.num[i]+rint.num[i];
        ret.num[i]=v%10;
        if(i+1>=100)continue;
        ret.num[i+1]+=v/10;

    }
    return ret;
}

char a[200],b[200];
int main(){
    int n;
    scanf("%d",&n);
    while(n--){
        scanf("%s%s",a,b);
        Integer ans=Integer(a)+Integer(b);
        int s=0;
        for(int i=0;i<100;i++)if(ans.num[i])s=i;
        if(s-1>=80){
            puts("overflow");
            continue;
        }
        for(int i=s;i>=0;i--)printf("%d",ans.num[i]);
        puts("");
    }
    return 0;
}