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 <cstdio>
#include <cstdlib>
#include <cstring>
#include <cmath>
#include <ctime>
#include <cassert>
#include <iostream>
#include <sstream>
#include <string>
#include <list>
#include <vector>
#include <queue>
#include <set>
#include <map>
#include <utility>
#include <numeric>
#include <algorithm>
#include <bitset>
#include <complex>
#include <fstream>
using namespace std;
typedef long long ll;
typedef vector<int> vint;
typedef pair<int, int> pint;
#define rep(i, n) REP(i, 0, n)
#define ALL(v) v.begin(), v.end()
#define MSG(a) cout << #a << " " << a << endl;
#define REP(i, x, n) for(int i = x; i < n; i++)
template<class T, class C> void chmax(T& a, C b){ if(a < b) a = b; }
template<class T, class C> void chmin(T& a, C b){ if(b < a) a = b; }
template<class T, class C> pair<T, C> mp(T a, C b){ return make_pair(a, b); }


const int lim = 80;

int main()
{
	int n;
	cin >> n;
	rep(i, n)
	{
		int ans[128];
		memset(ans, 0, sizeof(ans));
		string a, b;
		cin >> a >> b;

		int ai = a.size()-1, bi = b.size()-1;
		for(int i = 0; ai >= 0 || bi >= 0; ai--, bi--, i++)
		{
			int as = (ai >= 0 ? a[ai] - '0' : 0), bs = (bi >= 0 ? b[bi] - '0' : 0);

			ans[i] += as + bs;
			ans[i+1] += 9 < ans[i];
			if(9 < ans[i]) ans[i] -= 10;

			if(i == lim + 4) break;
		}


		if(ans[lim] != 0 ||(lim < a.size() || lim < b.size()))
		{
			cout << "overflow" << endl;
		}
		else
		{
			int index = 0;
			rep(i, lim)
				if(ans[i] != 0)
					index = i;

			for(int i = index; i >= 0; i--)
				cout << ans[i];

			cout << endl;
		}
	}
}

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

using namespace std;

int main(){
	int n,l1,l2,i;
	string a, b;
	char *ans;
	int ia,ib,sum;
	cin>>n;
	while(n--){
		cin>>a>>b;
		if(a.length()>80 || b.length()>80)cout<<"overflow"<<endl;
		if(a.length()==80 || b.length()==80){
			if((int)a[0]-'0'+(int)b[0]-'0'>9){
				cout<<"overflow"<<endl;
			}
		}
		if(a.length()<b.length()){
			swap(a,b);
		}
		l1=a.length();
		l2=b.length();
		ans=new char[l1+1];
		for(i=0;i<l1;i++)ans[i]='0';
		for(i=1;i<l2+1;i++){
			ia=(int)a[l1-i]-(int)'0';
			ib=(int)b[l2-i]-(int)'0';
			sum=ia+ib;
			ans[l1-i]+=sum;
		}
		for(i=l2+1;i<l1+1;i++){
			ans[l1-i]=a[l1-i];
		}
		for(i=0;i<l1;i++){
			if(ans[i]-'0'>9){
				ans[i]=ans[i]-10;
				ans[i-1]+=1;
			}
		}
		for(i=0;i<l1;i++){
			cout<<(int)ans[i]-'0';
		}
		cout<<endl;
	}
	while(1);
	return 0;
}

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

using namespace std;

int main(void){
	int n;

	cin >> n;
	for(int i=0;i<n;i++){
		int len,mlen=0;
		char val[2][81]={0},tmp[81],crr=0;

		for(int j=0;j<2;j++){
			cin >> tmp;
			len = strlen(tmp);
			mlen = mlen>len?mlen:len;
			for(int k=0;k<len;k++)
				val[j][len-1-k]=tmp[k]-0x30;
		}

		for(int j=0;j<mlen;j++){
			val[0][j]+=val[1][j]+crr;
			crr=val[0][j]/10;
			val[0][j]%=10;

			if(j==mlen-1&&crr)
				mlen++;
		}

		if(!val[0][80]){
			for(int j=0;j<mlen;j++)
				tmp[j]=val[0][mlen-1-j]+0x30;
			tmp[mlen]='\0';
		}
		else
			strcpy(tmp,"overflow");

		cout << tmp << 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';
		cout<<a<<endl;
		cout<<b<<endl;
		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]-'0')+(b[i]-'0')+f-10;
				f=1;
			}else{
				a[i]=(a[i]-'0')+(b[i]-'0');
				f=0;
			}
			a[i]+='0';
			cout<<a[i];
		}
		cout<<endl;
		reverse(a.begin(),a.end());
		cout<<endl<<f<<endl;
		if(f)a='1'+a;
		if(a.size()>80){
			cout<<"over flow"<<endl;
		}else{
			for(int i=0;i<a.size();i++){
				if(a[i]!='0' || i!=0)
				cout<<a[i];
			}
			cout<<endl;
		}
	}
	return 0;
}

C++: //0015
#include <bits/stdc++.h>
using namespace std;

int main(){
	long n;
	cin>>n;
	while(n--){
		string a,b,ans;
		long tmp = 0,i;
		cin>>a>>b;
		while(a.size() != b.size()){
		if(a.size() > b.size()){
			while(a.size() != b.size()){
				b = '0' + b;
			}
		}

		else if(a.size() < b.size()){
			while(a.size() != b.size()){
				a = '0' + a;
			}
		}
		}

		reverse(a.begin(),a.end());
		reverse(b.begin(),b.end());

		for(i = 0;i < a.size()||i < b.size();i++){
			tmp = (int)(a[i] - '0') + (int)(b[i] - '0') + (int)tmp;
			ans = ((char)((tmp % 10)+'0')) + ans;

			tmp /= 10;
		}

		if(tmp != 0)
			ans = '1' + ans;

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