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>
#define MAX 80

using namespace std;

int main()
{
	int n;
	cin >> n;
	for (int counter = 0; counter < n; counter++){
		string a, b;
		cin >> a >> b;
		if (a.length() > MAX || b.length() > MAX){
			cout << "overflow" << endl;
			continue;
		}
		if (a.length() == 1 && b.length() == 1 && a[0] == 0 && b[0] == 0){
			cout << 0 << endl;
			continue;
		}
		int inta[MAX + 2], intb[MAX + 2], sum[MAX + 2];
		fill(inta, inta + MAX + 2, 0);
		fill(intb, intb + MAX + 2, 0);
		fill(sum, sum + MAX + 2, 0);
		for (int i = 0; i < (signed)a.length(); i++){
			inta[i] = a[a.length() - i - 1] - '0';
		}
		for (int i = 0; i < (signed)b.length(); i++){
			intb[i] = b[b.length() - i - 1] - '0';
		}

		int over = 0;
		for (int i = 0; i < MAX + 2; i++){
			sum[i] = inta[i] + intb[i] + over;
			if (sum[i] >= 10){
				sum[i] %= 10;
				over = 1;
			}
			else{
				over = 0;
			}
		}

		for (int i = MAX + 1;; i--){
			if (sum[i] != 0){
				if (i >= MAX){
					cout << "overflow" << endl;
					break;
				}
				for (int j = i; j >= 0; j--){
					cout << sum[j];
				}
				cout << endl;
				break;
			}
		}
	}
	return 0;
}

C++: #include<iostream>
#include<string>
#include<vector>
int main(){
	std::string a,b;
	std::cin>>a>>b;
	std::string ans;
	if(a.size()<10&&b.size()<10){
		int A=0;
		int B=0;
		for(int i=a.size()-1;i>=0;i--)A+=a[i]-'0';
		for(int i=b.size()-1;i>=0;i--)B+=b[i]-'0';
		std::cout<<A+B<<std::endl;
	}else{
		int kr=0;
		int m=a.size()<=b.size()?a.size():b.size();
		int M=a.size()>b.size()?a.size():b.size();
		for(int i=m-1;i>=0;i--){
			int c=a[M],d=b[m];
			if(c+d>10)kr=1;
			int e=c+d;
			if(kr==1)e-=10;
			ans.push_back(e);
			a.erase(a.back()),b.erase(b.back());
			m--,M--;
		}
		ans.append(a);
	}
	std::cout<<ans<<std::endl;
	return 0;
}

C++: #include<iostream>
#include<string>
typedef unsigned int u;
using namespace std;
typedef string::iterator s;
u z=100000000;
void p(u*a,u*l,u n){
	if(a-l)
	p(a+1,l,(n+=*a*10)/z),
	*a=n%z;	
}
u t(u*a,u*l){
	u r=0;
	char b[11]={};
	if(a-l)
	if(!t(a+1,l)){
		if(*a)cout<<*a,r=1;
	}else
		sprintf(b,"%d",*a+z),
		cout<<b+1,
		r=1;
	return r;
}
void d(u*a,u*l,u*b,u n=0){
	if(a-l)
	d(a+1,l,b+1,(n+=(*a+*b))/z),
	*a=n%z;
}
int main(){
	u n;
	cin>>n;
	string a,b;
	getline(cin,a);
	for(;n--;){
		getline(cin,a);
		getline(cin,b);
		u x[11]={},y[11]={},c=0;
		for(s i=a.begin();i!=a.end();p(x,x+10,*i++-48))
			if(++c>80)goto g;
		c=0;
		for(s i=b.begin();i!=b.end();p(y,y+10,*i++-48))
			if(++c>80)goto g;
		d(x,x+11,y);
		if(x[10])goto g;
		else if(!t(x,x+10))cout<<'0';
		if(0)
g:
		cout<<"overflow";
		cout<<'\n';
	}
}

C++: #include<string>
#include<vector>
#include<iostream>
#include<algorithm>
#include<cstdio>
#include<cstdlib>
#include<sstream>
#include<functional>
#include<map>
#include<stack>
#include<set>
#include<cmath>

#define REP(i,s,e) for(int i=int(s);i<int(e);i++)
#define rep(i,n) for(int i=0;i<int(n);i++)

#define EPS 1e-6

using namespace std;

int main(){

	int n;
	cin >> n;

	rep(i,n){

	string a,b;
	cin >> a >> b;

	int sa=a.size(); int sb=b.size(); int p=max(sa,sb);
	vector<char>s;

	string z;
	if(sa>sb){
	rep(j,sa-sb)
	z+='0';
	b=z+b;
	}
	if(sb>sa){
	rep(j,sb-sa)
	z+='0';
	a=z+a;
	}


    int c=0;
	rep(j,p){

	c+=(a[p-1-j]-'0')+(b[p-1-j]-'0');

	s.push_back(c%10+'0');
	c=c/10;

	}

    if(c==1) s.push_back(c%10+'0');

    if(s.size()>80)
    cout << "overflow" << endl;
    else
    rep(k,s.size()) cout << s[s.size()-1-k];

    cout << endl;
    s.clear();

	}

	return 0;
}

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

using namespace std;

inline int toInt(string s) { int v; istringstream sin(s); sin>>v; return v; }
template<class T> inline string toString(T x){ ostringstream sout; sout<<x; return sout.str(); }

#define dump(x) cerr<<#x<<" = "<<(x)<<endl;
#define debug(x) cerr<<#x<<" = "<<(x)<<"(L"<<__LINE__<<")"<<" "<<__FILE__<<endl;

int main(){
	int N; cin >> N;
	for (int i = 0; i < N; i++) {
		string a, b; cin >> a >> b;
		if (a.size() < b.size()) {
			string tmp("");
			for (int j = 0; j < b.size()-a.size(); j++) { tmp += "0"; }
			a = tmp + a;
		} else if (b.size() < a.size()) {
			string tmp;
			for (int j = 0; j < a.size()-b.size(); j++) { tmp += "0"; }
			b = tmp + b;
		}
		
		string res("");
		for (int j = 0; j < a.size()+1; j++) res += "0";
		a = "0"+a; b = "0"+b;
		for (int j = res.size()-1; j >= 0; j--) {
			int cur = 0;
			if (res[j] >= '0' && res[j] <= '9') {
				cur = res[j]-'0';
			}
			int ai = a[j]-'0', bi = b[j]-'0';
			int res_tmp = ai+bi+cur;
			if (res_tmp >= 10) { 
				res[j] = '0'+(res_tmp%10);
				res[j-1] = '0'+(res_tmp/10); 
			} else {
				res[j] = '0'+res_tmp;
			}
		}
		int j = 0;
		while (res[j] == '0') { j++; }
		string ans("");
		for (; j < res.size(); j++) ans += res[j];
		
		if (ans.size() > 80) {
			cout << "overflow" << endl;
		} else {
			cout << ans << endl;
		}
	}

	return 0;
}