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++: /*
	0015:National Budget
*/
#include	<iostream>
#include	<string>
#include	<iterator>

using namespace std;

int main(void) {
	int n;
	
	cin >> n;
	for(int i = 0; i < n; i++) {
		string num1, num2, result;
		bool isMoveUp = false;
		
		cin >> num1 >> num2;
		string::reverse_iterator st1 = num1.rbegin();
		string::reverse_iterator st2 = num2.rbegin();
		while( st1 != num1.rend() || st2 != num2.rend()) {
			char buf1 = st1 != num1.rend() ? (*st1) : '0';
			char buf2 = st2 != num2.rend() ? (*st2) : '0';
			int buf3 = (buf1 - '0') + (buf2 - '0');
			
			isMoveUp = false;
			if(buf3 >= 10) {
				isMoveUp = true;
				(*(st1+1))++; buf3 -= 10;
			}
			result += (buf3 + '0');
			if(st1 != num1.rend()) { st1++; }
			if(st2 != num2.rend()) { st2++; }
		}
		if(isMoveUp) { result += '1'; }
		if(result.size() > 80) { cout << "overflou" << endl; continue; }
		string::reverse_iterator rt = result.rbegin();
		while(rt != result.rend()) {
			cout << (*rt);
			rt++;
		}
		cout << endl;
	}
	return 0;
}

C++: #include <stdio.h>
#include <string.h>
#define N 80

int main(void){
    char buf[N*2];
    int datas;
    int degits;
    int lint1[N+1], lint2[N+1], sum[N+1];
    int cry;
    
    scanf("%d", &datas);
    for (int n=0; n<datas; n++){
        // reset
        for (int i=0; i<N; i++) lint1[i]=lint2[i]=0;
        // input
        scanf("%s", buf);
        degits = strlen(buf);
        if (degits > 80){
         //   scanf("%s", buf); // read another num.
            puts("overflow");
            continue;
        }
        for (int i=0; i<degits; i++){
            lint1[degits-i-1]= buf[i]-'0';
        }
        scanf("%s", buf);
        degits = strlen(buf);
        if (degits > 80){
            puts("overflow");
            continue;
        }
        for (int i=0; i<degits; i++){
            lint2[degits-i-1]= buf[i]-'0';
        }
        // calc
        cry=0;
        for (int i=0; i<N; i++){
            sum[i] = lint1[i]+lint2[i] + cry;
            cry = sum[i]>9 ? 1:0;
            if (cry) sum[i] -= 10;
        }
        // output
        if (cry){
            puts("overflow");
        } else {
            bool zero=true;
            if (cry) puts("1");
            for (int i=N-1; i>=0; i--){
                if (zero && sum[i] != 0) zero=false;
                if (!zero) printf("%d", sum[i], i);
            }
            printf("\n");
        }
        
    }
    
    return 0;
}

C++: #include <iostream>
#include <cstring>
using namespace std;

int main(){
  int j;
  int n;
  char s1[111];
  char tmp[111];
  char s2[111];
  char ans[111];
  int  kuri = 0;
  cin >> n;
  while(n--){
  memset(ans,0,sizeof(ans));
  memset(s1,0,sizeof(s1));
  memset(s2,0,sizeof(s2));
  memset(tmp,0,sizeof(tmp));)
  cin >> s1 >> s2;
  if(strlen(s1) < strlen(s2)){
      for(int i=0,j=strlen(s1)-1;i<=j;i++,j--) swap(s1[i],s1[j]);
      for(int i=0,j=strlen(s2)-1;i<=j;i++,j--) swap(s2[i],s2[j]);
      for(int i=0;i<strlen(s2);i++){
	if((s2[i] + (s1[i]%'0') + kuri) > '9'){
	  ans[i] = (s2[i] + (s1[i]%'0') + kuri)-10;
	  kuri = 1;
	}
	else {
	  ans[i] = (s2[i] + (s1[i]%'0') + kuri);
	  kuri = 0;
	}
	if(i == strlen(s2)-1 && kuri == 1) ans[i+1] = '1';
      }
      if(strlen(ans) > 80) cout << "overflow" << endl;
      else{
	for(int i=strlen(ans)-1;i>=0;i--) cout << ans[i];
      }
  }
  else{
    for(int i=0,j=strlen(s1)-1;i<=j;i++,j--) swap(s1[i],s1[j]);
    for(int i=0,j=strlen(s2)-1;i<=j;i++,j--) swap(s2[i],s2[j]);
    for(int i=0;i<strlen(s1);i++){
      if((s1[i] + (s2[i]%'0') + kuri) > '9'){
	ans[i] = (s1[i] + (s2[i]%'0') + kuri)-10;
	kuri = 1;
      }
      else {
	ans[i] = (s1[i] + (s2[i]%'0') + kuri);
	kuri = 0;
      }
      if(i == strlen(s1)-1 && kuri == 1) ans[i+1] = '1';
    }
    if(strlen(ans) > 80) cout << "overflow" << endl;
    else{
      for(int i=strlen(ans)-1;i>=0;i--) cout << ans[i];
    }
  }
  cout << endl;
  }
}

C++: #include <string>
#include <vector>
#include <algorithm>
#include <iostream>
#include <cstdio>
#include <cstdlib>
#include <cmath>
#include <queue>
#include <map>
#define INF 100000000
#define rep(i,a) for(int i=0;i<(a);i++)

using namespace std;
typedef long long ll;






int main(){
    int n;
    cin>>n;
    
    rep(j,n){
        int x[100]={},y[100]={};
        string a,b;
        cin>>a>>b;
        if(a.size()>80 || b.size()>80){cout<<"overflow"<<endl;return 0;}
        else{
        for(int i=a.size()-1;i>=0;i--){
            x[a.size()-1-i]=a[i]-'0';
        }
        for(int i=b.size()-1;i>=0;i--){
            y[b.size()-1-i]=b[i]-'0';
        }
        int ans[100]={};
        for(int i=0;i<80;i++){
            
            ans[i]+=x[i]+y[i];
            ans[i+1]+=ans[i]/10;
            ans[i]-=ans[i]/10*10;
        }
        if(ans[80]!=0)cout<<"overflow"<<endl;
        else{
            bool flag=false;
            for(int i=79;i>=0;i--){
                if(ans[i]!=0)flag=true;
                if(flag||i==0)cout<<ans[i];
            }
        }
        cout<<endl;
        }
    }
    return 0;
}

C++: #include<stdio.h>
#include<math.h>
#include<string.h>

int main(void){

	int N,math1,math2,kuri,keta;
	int i,j,k;
	char fi[10000],se[10000],ans[10000];

	scanf("%d",&N);

	for(i=0;i<N;i++){

		scanf("%s",&fi);
		scanf("%s",&se);

		if(strlen(fi)<=80&&strlen(se)<=80){
		for(j=strlen(fi)-1,k=strlen(se)-1,keta=0,kuri=0;j>=0||k>=0;j--,k--,keta++){
			if(j<0) math1=0;
			else math1=fi[j]-'0';
			if(k<0) math2=0;
			else math2=se[k]-'0';
            ans[keta]=((math1+math2+kuri)%10)+'0';
			kuri=(math1+math2+kuri)/10;
		}
		if(kuri==1) printf("1");keta++;
		if(keta<=80){
			for(j=keta-1;j>=0;j--){
				printf("%c",ans[j]);
			}
		}
		else printf("overflow");
		}
		else printf("overflow");
		printf("\n");
	}

	return 0;
}

C++: #include <iostream>
#include <string>
 
using namespace std;
 
int main() {
    int N;
    cin >> N;
    while(N--) {
        string s1, s2;
        cin >> s1 >> s2;
 
        if(s1.size() > 80 || s2.size() > 80) {
            cout << "overflow" << endl;
            continue;
        }
        char buf[81] = {0};
        int p1 = s1.size()-1, p2 = s2.size()-1, pb = 79;
        int carry = 0;
        while(p2 >= 0) buf[pb--] = s2[p2--];
        pb = 79;
        while(pb >= 0 && (p1 >= 0 || carry > 0)) {
            int bd = buf[pb];
            if(bd > 0) bd -= '0';
            int bp = 0;
            if(p1 >= 0) bp = s1[p1--]-'0';
            int d = bp + bd + carry;
            carry = d / 10;
            d %= 10;
            buf[pb--] = d+'0';
        }
        while(pb >= 0 && buf[pb]) pb--;
        if(pb == -1 && carry > 0) cout << "overflow" << endl;
        else cout << (char*)(buf+(pb+1)) << endl;
    }
    return 0;
}