Go: package main

import (
        "fmt"
        "bufio"
        "os"
)

func main() {
        var n int
        var a, b string
        sc := bufio.NewScanner(os.Stdin)
        fmt.Scanf("%d", &n)
        for i := 0; i < n; i++ {
                sc.Scan()
                a = sc.Text()
                sc.Scan()
                b = sc.Text()

                if len(a) > len(b) {
                        s := add(a, b)
                        if s[len(a)] > 0 {
                                if len(a) >= 80 {
                                        fmt.Printf("overflow")
                                } else {
                                        for i := 0; i <= len(a);i++ {
                                                fmt.Printf("%d", s[len(a)-i])
                                        }
                                }
                                fmt.Printf("\n")
                        } else {
                                if len(a)-1 >= 80 {
                                        fmt.Printf("overflow")
                                } else {
                                        for i := 0; i < len(a);i++ {
                                                fmt.Printf("%d", s[len(a)-i-1])
                                        }
                                }
                                fmt.Printf("\n")
                        }
                } else {
                        s := add(b, a)
                        if s[len(b)] > 0 {
                                if len(b) >= 80 {
                                        fmt.Printf("overflow")
                                } else {
                                        for i := 0; i <= len(b);i++ {
                                                fmt.Printf("%d", s[len(b)-i])
                                        }
                                }
                                fmt.Printf("\n")
                        } else {
                                if len(b)-1 >= 80 {
                                        fmt.Printf("overflow")
                                } else {
                                        for i := 0; i < len(b);i++ {
                                                fmt.Printf("%d", s[len(b)-i-1])
                                        }
                                }
                                fmt.Printf("\n")
                        }
                }
        }
}

// expects ln > lm
func add(n string, m string) (s [100]int) {
        ln := len(n)
        lm := len(m)
        var sum [100]int

        for i := 0; i < ln; i++ {
                var s, t int
                s = int(n[ln-1-i]) - int('0')

                if lm - 1 - i >= 0 {
                        t = int(m[lm-1-i]) - int('0')
                } else {
                        t = 0
                }

                sum[i+1] = (s + t + sum[i]) / 10
                sum[i] = (s + t + sum[i]) % 10
        }

        return sum
}

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 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]=in[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){
			if(m_len>79){
				cout<<"overflow"<<endl;
				continue;
			}
			cout<<ans[m_len];
		}/*
		for(int i=0;i<m_len;i++){
			cout<<ans[m_len-1-i];
		}cout<<endl;
		
		*/
	}
	return 0;
}

C++: #include <iostream>
#include <algorithm>
#include <vector>
#include <cstdio>
#include <string>
#include <cmath>
#include <cfloat>
#include <map>
using namespace std;
int number(char c){
	if(c>='0'&&c<='9')
	return (c-'0');
	else
		return 0;
}
int main(){
	int n;
	cin>>n;
	
	while(n>0){
		string a;
		string b;
	int k=0;
	int c=0;
	string s="";
	
		cin>>a;
		cin>>b;
		
	
	for(int i=a.length()-1,j=b.length()-1;i>=0||j>=0;i--,j--){
		int d;
		int n1=0,n2=0;
		if(i>=0)
			n1=number(a[i]);
		if(j>=0)
			n2=number(b[j]);
		d=n1+n2+k;
		k=d/10;
		d=d%10;
		c++;
		s=to_string(d)+s;
	}
	if(c>=80&&k>0)
		cout<<"overflow"<<endl;
		else{
			if(k>0){
		s=to_string(k)+s;
			}
		cout<<s<<endl;
		}
		n--;
	}
}

C++: #include <iostream>
#include <string>
using namespace std;
int main(){
	int n;
	cin >> n;
	while(n--){
		string a, b;
		bool cu = false;
		cin >> a >> b;
		int al = a.length(), bl = b.length(), ans[101] = {0};
		for(int i = 0; i < min(al, bl); i++){
			ans[i] = a[al-i-1] + b[bl-i-1] - 48 * 2;
			if(cu) ans[i]++;
			if(ans[i] >= 10){
				ans[i] %= 10;
				cu = true;
			}else cu = false;
		}
		for(int i = min(al, bl); i < max(al, bl); i++){
			if(al > bl) ans[i] = a[al-i-1] - 48 + cu;
			else ans[i] = b[bl-i-1] - 48 + cu;
			if(ans[i] >= 10){
				ans[i] %= 10;
				cu = true;
			}else cu = false;
		}
		if(cu) ans[max(al, bl)] = 1;
		if(max(al, bl)+cu > 80) cout << "overflow";
		else for(int i = (cu? max(al, bl):max(al, bl)-1); i >= 0; i--) cout << ans[i];
		cout << endl;
	}
	return 0;
}

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

int main(){
	int n;
	cin >> n;
	while( n-- > 0 ){
		string num1, num2;
		cin >> num1 >> num2;
		char sum[101];
		int degit = 0;
		/*
		for( int i = 0; i < 101; i++ ) sum[i] = '0';
		for( int i = 0; i < num1.size(); i++ ){
			sum[100-i] = num1[num1.size()-i-1];
		}
		int d;
		for( int i = 0; i < num1.size() || i < num2.size() || degit != 0; i++ ){
			int now;
			if( i < num2.size() ){
				now = (int)(sum[100-i]-'0') + (int)(num2[num2.size()-i-1]-'0') + degit;
			}else{
				now = (int)(sum[100-i]-'0') + degit;
			}
			sum[100-i] = (char)(now%10)+'0';
			if( now >= 10 ) degit = 1;
			else degit = 0;
			d = i;
		}
		*/
		int d = 3;
		if( d < 80 ){
			for( int i = 100-d; i < 101; i++ ) cout << sum[i];
			cout << endl;
		}else{
			cout << "overflow" << endl;
		}
	}
}

C++: #include<iostream>
#include<algorithm>
#include<cstring>
#include<string>
#define MOJI 48
using namespace std;
int multiple(string a,string b);
char c[111111];
int main(){
  int n;
  string a,b;
  cin >> n;
  for(int i=0;i<n;i++){
    cin >> a;
    cin >> b;
    int k = multiple( a , b );
    if(k>=80){
      cout << "overflow" << endl;
    }else {
      for(int i=k-1;i>=0;i--){
	printf("%d",c[i]);
      }
      cout << endl;
    }
  }
}
int multiple(string a,string b){
  int a_size,b_size,w=0;
  char k = 0; // 桁上げ
  a_size = a.size();
  b_size = b.size();
  while(a_size>0 || b_size>0 || k == 1){
    if(a_size>0 && b_size>0){
      c[w] = (a[a_size-1] - MOJI + b[b_size-1] - MOJI + k) % 10;
      k = (k + (a[a_size-1]-MOJI) + (b[b_size-1]-MOJI)) / 10;
    }
    else if(a_size>0 && b_size<=0){
      c[w] = ((a[a_size-1]-MOJI) + k) % 10;
      k = (a[a_size-1]-MOJI + k) / 10;
    }
    else if(a_size<=0 && b_size>0){
      c[w] = (b[b_size-1] - MOJI + k) % 10;
      k = (k + b[b_size-1] - MOJI) / 10;
    }else {
      c[w] = k;
      k = 0;
    }
    a_size--;
    b_size--;
    w++;
  }
  return w;
}