Go: package main

import (
	"bufio"
	"fmt"
	"math/big"
	"os"
	"strconv"
)

var sc = bufio.NewScanner(os.Stdin)

func main() {
	sc.Scan()
	dataset, _ := strconv.Atoi(sc.Text())
	var firstdata *big.Int

	for i := 0; i < dataset*2; i++ {
		n := new(big.Int)
		sc.Scan()
		data, _ := n.SetString(sc.Text(), 10)

		if i%2 == 0 {
			firstdata = data
			continue
		}

		result := new(big.Int)
		if result := result.Add(data, firstdata); len(result.String()) > 80 {
			fmt.Println("overflow")
			continue
		} else {
			fmt.Println(result)
		}
	}
}

C++: #include <iostream>
#include <string>
#include <vector>
#include <algorithm>
#include <cmath>
#include <cstdio>
#include <functional>
#include <numeric>
#include <stack>
#include <queue>
#include <map>
#include <set>
#include <utility>
#include <sstream>
#include <complex>

using namespace std;

#define FOR(i,a,b) for(long long i=(a);i<(b);i++)
#define REP(i,N) for(long long i=0;i<(N);i++)
#define ALL(s) (s).begin(),(s).end()
#define fi first
#define se second

#define PI acos(-1.0)
#define INF 1000000007

typedef long long ll;
typedef pair<ll, ll> P;
typedef pair<string, ll> PS;
typedef vector<ll> V;

int k = 0, n;
string s, t;
vector<string> ans;

int main(){
	cin >> n;
	while (cin >> s >> t&&n){
		n--;
		k = 0;
		if (s.size() >= 80 || t.size() >= 80){
			cout << "overflow" << endl;
		}
		reverse(ALL(s));
		reverse(ALL(t));
		int a = max(s.size(), t.size()), b = min(s.size(), t.size());
		if (s.size() > t.size()){
			REP(i, a - b){
				t += "0";
			}
		}
		else{
			REP(i, a - b){
				s += "0";
			}
		}		
		reverse(ALL(s));
		reverse(ALL(t));
		cout << s << endl;
		cout << t << endl;
		REP(i, s.size()){
			int x = (s[s.size()-1-i] - '0') + (t[s.size()-1-i] - '0');
			string q;
			stringstream ss;
			if (i != s.size() - 1){
				ss << ((x + k) % 10);
			}
			else{
				ss << x + k;
			}
			ss >> q;
			ans.push_back(q);
			if (x + k >= 10){
				k = (x + k) / 10;
			}
			else{
				k = 0;
			}
		}
		reverse(ALL(ans));
		REP(i, ans.size()){
			cout << ans[i];
		}
		cout << endl;
		ans.clear();
	}
}

C++: #define scanf_s scanf
#define sscanf_s sscanf
#include <stdio.h>
#include <string>
#include <iostream>
#include <math.h>
using namespace std;
#define MAX 81
#define _MAX 100
//#define CONSTNUM 10000000
int main(void)
{
	int n, s = 0, x, y, r = 0, a, b;
	int sum_n[50][3] = { 0 };
	char sum[50][_MAX], astr[_MAX], bstr[_MAX], num[11] = { "0123456789" };
	char ca, cb;
	scanf_s("%d", &n);
	for (int i = 0; i < n; ++i) {
		scanf_s("%s %s", &astr, &bstr);
		for (x = 0; astr[x] != '\0'; ++x);
		for (y = 0; bstr[y] != '\0'; ++y);
		if (x >= 80 || y >= 80) {
			printf("overflow\n");
			continue;
		}
		for (int j = 0; j < x || j < y; ++j) {
			if (j < x && j < y) {
				ca = astr[x - j - 1]; cb = bstr[y - j - 1];
				sscanf_s(&ca, "%d", &a);
				sscanf_s(&cb, "%d", &b);
				if (x > y) sum[i][x - j] = num[(a + b + r) % 10];
				else sum[i][y - j] = num[(a + b + r) % 10];
				if (a + b + r >= 10) r = (a + b + r) / 10;
				else r = 0;
			}
			else {
				if (x > y) {
					if (r != 0) {
						ca = astr[x - j - 1];
						sscanf_s(&ca, "%d", &a);
						sum[i][x - j] = num[(a + r) % 10];
						if (a + r >= 10) r = (a + r) / 10;
						else r = 0;
					}
					else {
						sum[i][x - j] = astr[x - j - 1];
					}
				}
				else {
					if (r != 0) {
						cb = bstr[y - j - 1];
						sscanf_s(&cb, "%d", &b);
						sum[i][y - j] = num[(b + r) % 10];
						if (b + r >= 10) r = (b + r) / 10;
						else r = 0;
					}
					else {
						sum[i][y - j] = bstr[y - j - 1];
					}
				}
			}
		}
		if (y < 80 - r && x < 80 - r) {
			if (r == 1) 	printf("%d", r);
			for (int j = 1; j <= x || j <= y; ++j) {
				printf("%c", sum[i][j]);
			}
		}
		else {
			printf("overflow");
		}
		printf("\n");
		r = 0;
	}
}

C++: #include <iostream>
#include <string>
#include <algorithm>
using namespace std;
int main(){

  int n;
  cin>>n;
  for(int i=0;i<n;i++){
    string a,b,s;
    int how=0;
    cin>>a>>b;
    reverse(a.begin(),a.end());
    reverse(b.begin(),b.end());
    for(int j=0;j<a.size()||j<b.size();j++){
      if(j<a.size()) how+=a[j]-'0';
      if(j<b.size()) how+=b[j]-'0';
      s.push_back(how%10+'0');
      how/=10;
    }
    if(how!=0) s.push_back(how+'0');
    reverse(s.begin(),s.end());
    if(s.size()>80) cout<<"overflow"<<endl;
    else cout<<s<<endl;
  }
  return 0;

}

C++: #include<iostream>
#include<vector>
#include<string>
using namespace std;
vector<int> V,W,A;
int main(){
	int n,a,b;
	cin>>n;
	string str;
	char num[10]={'0','1','2','3','4','5','6','7','8','9'};
	for(int i=0;i<n;i++){
		cin>>str;
		for(int j=0;j<str.size();j++){
			for(int k=0;k<10;k++){
				if(str[j]==num[k]){
					V.push_back(k);
					break;
				}
			}
		}
		cin>>str;
		for(int j=0;j<str.size();j++){
			for(int k=0;k<10;k++){
				if(str[j]==num[k]){
					W.push_back(k);
					break;
				}
			}
		}
		int v=V.size(),w=W.size();
		int flag=0;
		while(v || w){
			int x=0;
			if(v==0){
				if(W[w-1]+flag>=10){
					A.push_back(W[w-1]+flag-10);
					W.pop_back();
					flag=1;
				}else{
					A.push_back(W[w-1]+flag);
					W.pop_back();
					flag=0;
				}
			}else if(w==0){
				if(V[v-1]+flag>=10){
					A.push_back(V[v-1]+flag-10);
					V.pop_back();
					flag=1;
				}else{
					A.push_back(V[v-1]+flag);
					V.pop_back();
					flag=0;
				}
			}else{
				if(V[v-1]+W[w-1]+flag>=10){
					A.push_back(V[v-1]+W[w-1]-10+flag);
					V.pop_back();
					W.pop_back();
					flag=1;
				}else{
					A.push_back(V[v-1]+W[w-1]+flag);
					V.pop_back();
					W.pop_back();
					flag=0;
				}
			}
			v=V.size();
			w=W.size();
		}
		if(flag)
		A.push_back(flag);
		for(int j=A.size()-1;j>=0;j--){
			cout<<A[j];
		}
		cout<<endl;
		V.clear();
		W.clear();
		A.clear();
	}
	return 0;
}

C++: #include <cstdio>
#include <cstring>
using namespace std;
#define def 1000

int main(void){
  int i,j,n,len,tmp,max,digits,A[def],B[def],C[def];
  char str[def];

  fgets(str,sizeof(str),stdin);
  sscanf(str,"%d",&n);
  for(i=0;i<n;i++){
    memset(A,0,sizeof(A));
    memset(B,0,sizeof(B));
    memset(C,0,sizeof(C));
    tmp = 0;
    digits = 0;
    /*for(j=0;j<5;j++){
      printf("%d",A[j]);
    }
    printf("\n");*/
    fgets(str,sizeof(str),stdin);
    len = strlen(str)-2;
    max = len;
    digits = max+1;
    //printf("%d\n",len);
    for(j=len;j>=0;j--){
      A[len-j] = str[j]%48;
      //printf("%d",A[len-j]);
    }
    //printf("\n");
    fgets(str,sizeof(str),stdin);
    len = strlen(str)-2;
    if(len>max){
      max = len;
      digits = max+1;
    }
    for(j=len;j>=0;j--){
      B[len-j] = str[j]%48;
      //printf("%d",A[len-j]);
    }
    for(j=0;j<=max;j++){
      //printf("%d ",(A[j] + B[j] + tmp)%10);
      C[max+1-j] = (A[j] + B[j] + tmp)%10;
      tmp = (A[j] + B[j] + tmp)/10;
      //printf("%d\n",tmp);
      if(j==max && tmp!=0){
        C[0] = tmp;
        digits += 1;
      }
      if(digits>80){
        printf("overflow");
        goto end;
      }
      //printf("%d",B[len-j]);
    }
    for(j=0;j<=max+1;j++){
      if(j==0 && C[j]==0) continue;
      printf("%d",C[j]);
    }
    end:;
    printf("\n");
    //Calculation(A,B);
  }
  return 0;
}