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 "bits/stdc++.h"

#define debug(x) cout<<#x<<": "<<x<<endl
#define rep(i,n) for (int i=0;i<(n);i++)
#define FOR(i,a,b) for (int i=(a);i<=(b);i++)
#define all(a) (a).begin(),(a).end()
using namespace std;
typedef vector<int> VI;
typedef vector<vector<int>> VVI;
typedef long long ll;

void solve() {
#ifdef _WIN32
	istream &cin = ifstream("input.txt");
#endif
	int n;
	cin >> n;
	rep(i, n) {
		vector<int> v1(101), v2(101);
		string s1, s2;
		cin >> s1 >> s2;
		if (s1.size() > 80 || s2.size() > 80) {
			cout << "overflow" << endl;
			return;
		}

		rep(j, s1.size()) v1[j] = s1[s1.size() - j - 1] - '0';
		rep(j, s2.size()) v2[j] = s2[s2.size() - j - 1] - '0';
		
		rep(j, 100) {
			v1[j] += v2[j];
			if (v1[j] >= 10) {
				v1[j] -= 10;
				v1[j + 1]++;
			}
		}
		int digit = 0;
		for (int j = 100; j >= 0; j--) {
			if (digit == 0 && v1[j]) {
				digit = j;
				break;
			}
		}
		if (digit >= 80) cout << "overflow" << endl;
		else {
			for (int j = digit; j >= 0; j--) cout << v1[j];
			cout << endl;
		}
	}

}




int main() {
	cin.tie(0);
	ios::sync_with_stdio(false);


	solve();
	//system("PAUSE");
	return 0;
}

C++: #include <bits/stdc++.h>

using namespace std;

#define FOR(i,a,b) for(int i=(a);i<(b);i++)
#define REP(i,a) FOR(i,0,a)

vector<int> a(200);
vector<int> b(200);
vector<int> comp(200);

void dfs(int i){
	//i?????????1?¶????
	if (comp[i]<9){
		comp[i]++;
	}
	else{
		comp[i]=0;
		dfs(i+1);
	}
}

int main(){
	int N;
	cin>>N;
	REP(i,N){
		string s1,s2;
		REP(j,200){
			a[j]=b[j]=0;
		}
		cin>>s1;
		cin>>s2;
		REP(j,s1.size()){
			comp[j]=s1[s1.size()-j-1]-'0';
		}
		REP(j,s2.size()){
			b[j]=s2[s2.size()-j-1]-'0';
		}
		REP(j,s2.size()){
			comp[j]+=b[j];
			if (comp[j]>9){
				comp[j]-=10;
				dfs(j+1);
			}
		}
		int fl=200-1;
		while(!comp[fl]) fl--;
		fl++;
		if (fl>80){
			cout<<"overflow"<<endl;
		}
		else{
			for(int k=fl-1;k>=0;k--){
				cout<<comp[k];
			}
			cout<<endl;
		}
	}
	return 0;
}

C++: #include<iostream>
#include<algorithm>
#include<vector>
#include<stack>
#include<queue>
#include<cstdio>
#include<climits>
#include<cmath>
#include<cstring>
#include<string>
#include<complex>
#include<iomanip>

#define EPS (1e-10)
using namespace std;
#define PI (3.1415926535)

int main(){
	string a, b;
	vector<int> m, n, s;
	int set;

	m.resize(81, 0);
	n.resize(81, 0);
	s.resize(81, 0);

	cin >> set;
	while(set-- > 0){
		cin >> a >> b;
		for(int i = 0; i < 81; i++){
			m[i] = n[i] = s[i] = 0;
		}		
		if(a.size() > 80 || b.size() > 80){
			cout << "overflow" << endl;
		}else{
			reverse(a.begin(), a.end());
			reverse(b.begin(), b.end());
			for(int i = 0; i < a.size(); i++){
				m[i] = a[i] - '0';
			}
			for(int i = 0; i < b.size(); i++){
				n[i] = b[i] - '0';
			}
			for(int i = 0; i < 80; i++){
				s[i] += m[i] + n[i];
				if(s[i] >= 10){
					s[i+1]++;
					s[i]-=10;
				}
			}
			int digit = 81;
			while(--digit > 0){
				if(s[digit] != 0){
					break;
				}
			}
			if(digit >= 80){
				cout << "overflow" << endl;
			}else{
				for(int i = digit; i >= 0; i--){
					cout << s[i];
				}
				cout << endl;
			}
		}
	}

	return 0;
}

C++: #include<iostream>
#include<stdio.h>
#include <algorithm>
#include <utility>
using namespace std;
using namespace std;
int main(){
	
	int m;
	char in[2][100];
	char ans[100];
	int ie[2];
	int ien[2];
	bool max10;
	int Min;
	bool NMin;
	scanf("%d",&m);
	for(;m--;){
		
		for(int i=0;i<2;i++){
			scanf(" %s",&in[i]);
		}
		ie[0]=strlen(in[0])-1;
		ie[1]=strlen(in[1])-1;
		Min=max(ie[0],ie[1]);
		if((Min>79)||((Min>78)&&(ie[0]==ie[1])&&((int(in[0][0])+int(in[1][0]))>9))){
			cout<<"overflow"<<endl;
		}else{	
			if(ie[0]>ie[1]){
				NMin=0;
			}else{
				NMin=1;
			}
			ien[0]=ie[0];
			ien[1]=ie[1];
			for(;min(ien[0],ien[1])+1;ien[0]--,ien[1]--){
				in[NMin][ien[NMin]]=in[0][ien[0]]+in[1][ien[1]]-'0';
				if(in[NMin][ien[NMin]]>9+'0'){
					in[NMin][ien[NMin]]-=10;
					in[NMin][ien[NMin]-1]  +=1;
				}
			}
			max10=0;
			for(;ien[NMin]>-1;ien[NMin]--){
				if(in[NMin][ien[NMin]]>9+'0'){
					in[NMin][ien[NMin]]-=10;
					in[NMin][ien[NMin]-1]+=1;
					max10=1;
				}else{
					max10=0;
				}
			}
			if(max10) cout<<"1";
			cout<<in[NMin]<<endl;
		}
	}
	return 0;
}