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 <string>
#include <math.h>
#include <algorithm>
using namespace std;

int main()
{
	int n;
	cin >> n;
	
	for(int j = 0; j < n; ++j)
	{
		string numA;
		string numB;
		cin >> numA >> numB;
		/*
		if(numA.size()<numB.size())swap(numA,numB);
		while(numA.size()>numB.size())numB="0"+numB;
		*/
		while(numA.size() != numB.size())
		{
			if(numA.size() < numB.size())
			{
				numA = "0" + numA;
			}
			else
			{
				numB = "0" + numB;
			}
		}
		
		numA = "0" + numA;
		numB = "0" + numB;
		
		for(int i = numA.size() - 1; i >= 0; --i)
		{
			int sum = 0;
			sum = (numA[i] - '0') + (numB[i] - '0');
			
			if(sum >= 10)
			{
				sum -= 10;
				numA[i - 1] += 1;
			}
			
			numA[i] = sum + '0';
		}
		
		if(numA.size() > 80)
		{
			cout << "overflow" << endl;
			continue;
		}
		if(numA.size() == 81 && numA[0] != '0')
		{
			cout << "overflow" << endl;
			continue;
		}
		
		for(int i = 0; i < numA.size(); ++i)
		{
			if(i == 0 && numA[i] == '0') continue;
			cout << numA[i];
		}
		
		cout << endl;
	}
	return 0;
}

C++: #include<iostream>
#include<algorithm>
#include<sstream>
#include<string>
using namespace std;
int main(){
	int n;
	for(cin>>n;n--;){
		string a,b;
		cin>>a;
		cin>>b;
		reverse(a.begin(),a.end());
		reverse(b.begin(),b.end());
		while(a.size()<b.size())a+='0';
		while(b.size()<a.size())b+='0';
		a+='0';
		int f=0;
		for(int i=0;i<a.size()-1;i++){
			if((a[i]-'0')+(b[i]-'0')+f>9){
				a[i]=a[i]+(b[i]-'0')+f-10;
				f=1;
			}else{
				a[i]=a[i]+(b[i]-'0');
				f=0;
			}
		}
		reverse(a.begin(),a.end());
		if(f)a[0]='1';
		if(a.size()>80){
			cout<<"overflow"<<endl;
		}else{
			for(int i=0;i<a.size();i++){
				if(a[i]!='0' || i!=0)
				cout<<a[i];
			}
			cout<<endl;
		}
	}
	return 0;
}

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

int main(void){
	int n;
	std::cin >> n;
	char i1[81]="",i2[81]="",res[81]="";
	for(int i=0; i<n; i++){
		bool cflag=false;
		std::string tmp;
		std::cin >> tmp;
		if(tmp.size()>80){
			cflag=true;break;
		}
		strcpy(i1,tmp.c_str());
		std::cin >> tmp;
		if(tmp.size()>80){
			cflag=true;break;
		}
		strcpy(i2,tmp.c_str());
		for(int j=0;j<80;j++){
			char c='0';
			if(cflag)
				c+=1;
			cflag=false;
			if(strlen(i1)>j){
				c+=i1[strlen(i1)-j-1]-'0';
			}
			if(strlen(i2)>j){
				c+=i2[strlen(i2)-j-1]-'0';
			}
			if(c > '9'){
				cflag=true;
				c-=10;
			}
			res[79-j]=c;
		}

		if(cflag){
			std::cout << "overflow" << std::endl;
		}else{
			char *p = res;
			while(*p++ == '0');
			std::cout << (p-1) << std::endl;
		}
	}
	return 0;
}

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

int main(void){
  string x,y;
  int tmp, c;
  int N;
  int cnt;
  cin >> N;

  for(int i = 0; i < N; i++){
    stack<int> a, b, ans;
    cnt = 0;

    cin >> x;
    cin >> y;

    for(int j = 0; j < x.length(); j++){
      a.push(x[j] - '0');
    }
    for(int j = 0; j < y.length(); j++){
      b.push(y[j] - '0');
    }

    c = 0;

    while(a.empty() != true || b.empty() != true){
      tmp = 0;
      
      if(a.empty() != true){
	tmp += a.top();
	a.pop();
      }
      if(b.empty() != true){
	tmp += b.top();
	b.pop();
      }
      tmp += c;

      ans.push(tmp % 10);
      cnt++;

      if(tmp >= 10)
	c = tmp / 10;
      else
	c = 0;
    }

    if(c != 0){
      ans.push(c);
      cnt++;
    }

    if(cnt > 80)
      cout << "overflow" << endl;
    else{
      while(ans.empty() != true){
	cout << ans.top();
	ans.pop();
      }
      cout << endl;
    }
  }
  return 0;
}

C++: #include <stdio.h>

#define DEGITS 8

void load(int a[]);
void print(int a[]);

int main(void){
    int a[DEGITS]; // a[0] = LSD
    int b[DEGITS];
    int n;
    int cry;
    
    scanf("%d", &n);
    for (int i=0; i<n; i++){
        // reset
        for (int i=0; i<DEGITS; i++){
            a[i]=b[i]=0;
        }
        // load
        load(a);
        load(b);
        // add ( a <- a + b )
        cry=0;
        for (int d=0; d<DEGITS; d++){
            a[d] += (b[d] + cry);
            cry = a[d]/10;
            if (a[d]>9){
                a[d]-=10;
            }
        }
        // print
        if (cry){
            printf("overflow\n");
        } else{
            print(a);
        }
    }

    return 0;
}

void load(int a[]){
    char *pt;
    char st[DEGITS+1];
    
    scanf("%s", st);
    for (pt=st; *pt!='\0'; pt++) ;
    for (int i=0; pt!=st; i++){
        a[i] = *(--pt)-'0';
    }
}

void print(int a[]){
    int d;
    
    for (d=DEGITS-1; d>0; d--){ 
        if (a[d] != 0) break;
    }
    for (; d>0; d--) printf("%d", a[d]);
    printf("%d\n", a[0]);
}