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<algorithm>
#include<string>
using namespace std;
int main() {
	int n;
	cin >> n;
	string k, l;
	for(int j=0;j<n;j++) {
		int c = 0, d;
		cin >> k >> l;
		string g = "";
		string a="", b="";
		for (int i = k.length() - 1; i >= 0; i--) 
			a += k[i];
		for (int i = l.length()-1; i >= 0; i--)
			b += l[i];
		for (int i = 0; i < max(a.length(), b.length()) + 1; i++) {
			if (i < min(a.length(), b.length())) {
				int e = (int)a[i] - '0', f = (int)b[i] - '0';
				d = e + f + c;
				if (d >= 10) {
					g += (char)(d - 10) + '0';
					c = 1;
				}
				else {
					g += (char)d + '0';
					c = 0;
				}
			}
			else if (i < a.length()) {
				int e = (char)a[i] - '0';
				e += c;
				if (e >= 10) {
					g += (char)(e - 10) + '0';
					c = 1;
				}
				else {
					g += (char)e + '0';
					c = 0;
				}
			}
			else if (i < b.length()) {
				int e = (char)b[i] - '0';
				e += c;
				if (e >= 10) {
					g += (char)(e - 10) + '0';
					c = 1;
				}
				else {
					g += (char)e + '0';
					c = 0;
				}
			}
			else if (c == 1) {
				g += (char)c + '0';
				c = 0;
			}
		}
		string h = "";
		if (g.length() >= 81)
			cout << "overflow" << endl;
		else {
			for (int i = g.length() - 1; i >= 0; i--)
				h +=g[i];
			cout << h << endl;
		}
	}
}

C++: #include<iostream>
#include<string>
using namespace std;
int main(){
 string s1,s2;
 int a1[100],a2[100],i,n,l1,l2,l;
 cin>>n;
 while(n--){
   cin>>s1>>s2;
   l1=s1.size();
   l2=s2.size();
   for(i=l1-1;i>-1;i--)
     a1[i]=int(s1[l1-i-1])-int('0');
   for(i=l2-1;i>-1;i--)
     a2[i]=int(s2[l2-i-1])-int('0');
   if(l1>l2)
     l=l1;
   else
     l=l2;
   for(i=0;i<l;i++){
     a1[i]=(a1[i]+a2[i])%10;
     a1[i+1]+=(a1[i]+a2[i])/10;
   }
   if (a1[l]==1)
    ++l;
   if (l<=80){
     for(i=l-1;i>-1;i--)
       cout<<a1[i];
       cout<<endl;
   }
   else {
     cout<<"overflow"<<endl;
     continue;
   }
 }
 return 0;
}

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

using namespace std;

typedef long long ll;

const ll BASE = 10000;
const int LogB = 4;
const int MAX_DIGIT = 100;

struct Bigint {
	ll digit[MAX_DIGIT];
	int size;
	Bigint (int sz = 1, ll a = 0) : size(sz) {
		memset(digit, 0, sizeof(digit));
		digit[0] = a;
	}
};

const Bigint ZERO(1,0), ONE(1, 1);

bool operator < (Bigint x, Bigint y) {
	if (x.size != y.size) return x.size < y.size;
	for (int i=x.size-1; i>=0; i--) if (x.digit[i] != y.digit[i]) return x.digit[i] < y.digit[i];
	return false;
}

bool operator > (Bigint x, Bigint y) { return y < x; }
bool operator <= (Bigint x, Bigint y) { return !(y < x); }
bool operator >= (Bigint x, Bigint y) { return !(x < y); }
bool operator != (Bigint x, Bigint y) { return x < y || y < x; }
bool operator == (Bigint x, Bigint y) { return !(x < y) && !(y < x); }

Bigint normal(Bigint x) {
	ll c = 0;
	for (int i=0; i<x.size; i++) {
		while (x.digit[i] < 0)
			x.digit[i+1] -= 1, x.digit[i] += BASE;
		ll a = x.digit[i] + c;
		x.digit[i] = a % BASE;
		c = a / BASE;
	}
	for (; c>0; c /= BASE) x.digit[x.size++] = c % BASE;
	while (x.size > 1 && x.digit[x.size-1] == 0) x.size--;
	return x;
}

Bigint convert(ll a) { return normal(Bigint(1, a)); }

Bigint convert(const string& s) {
	Bigint x;
	int i = s.size() % LogB;
	if (i > 0) i -= LogB;
	for (; i<(int)s.size(); i += LogB) {
		ll a = 0;
		for (int j=0; j<LogB; j++) a = 10 * a + (i+j >= 0 ? s[i+j] - '0' : 0);
		x.digit[x.size++] = a;
	}
	reverse(x.digit, x.digit+x.size);
	return normal(x);
}

ostream &operator << (ostream& os, Bigint x) {
	os << x.digit[x.size-1];
	for (int i=x.size-2; i>=0; i--) os << setw(LogB) << setfill('0') << x.digit[i];
	return os;
}

istream &operator >> (istream& is, Bigint &x) {
	string s; is >> s;
	x = convert(s);
	return is;
}

Bigint operator + (Bigint x, Bigint y) {
	if (x.size < y.size) x.size = y.size;
	for (int i=0; i<y.size; i++) x.digit[i] += y.digit[i];
	return normal(x);
}

int main() {
	int n;
	cin >> n;
	string s1, s2;
	
	for (int i=0; i<n; i++) {
		cin >> s1;
		cin >> s2;
		Bigint x = convert(s1), y = convert(s2);
		Bigint z = x + y;
		if (z.size > 80) cout << "overflow" << endl;
		else cout << z << endl;
	}
	
	return 0;
}