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<cstdio>
#include<stack>
#include<algorithm>
using namespace std;
string str1;
string str2;
stack<int> sta;

int main()
{
  int n;
  int i,j;
  int exc;//??°?????????
  int sum;

  cin>>n;
  for(i=0;i<n;i++){
    cin>>str1;
    cin>>str2;
    if(str1.size()>=80 || str2.size()>=80){
      cout<<"overflow"<<endl;
      return 0;
    }
    exc=0;
    for(j=1;j<=min(str1.size(),str2.size());j++){
      sum=str1[str1.size()-j]-'0'+str2[str2.size()-j]-'0'+exc;
      exc=sum/10;
      sta.push(sum%10);
    }
    if( str1.size() < str2.size() ){
      while(j<=str2.size()){
	sum=str2[str2.size()-j]-'0'+exc;
	exc=sum/10;
	sta.push(sum%10);
	j++;
      }
    }else if( str1.size() > str2.size() ){
      while(j<=str1.size()){
	sum=str1[str1.size()-j]-'0'+exc;
	exc=sum/10;
	sta.push(sum%10);
	j++;
      }
    }
    if(max(str1.size(),str2.size())==79 && exc==1){
      cout<<"overflow"<<endl;
      return 0;
    }
    if(exc)
      cout<<exc;
    for(j=0;j<max(str1.size(),str2.size());j++){  
      cout<<sta.top();
      sta.pop();
    }
    cout<<endl;
  }
}

C++: #include <cstdio>
#include <cmath>
#include <cstdlib>
#include <cstring>

const int MAX_INPUT = 1000000;
const int ARRAY_SIZE = 20;
const int MAX_LEN = 8;
char inputA[MAX_INPUT];
char inputB[MAX_INPUT];

void toBigInt(char *str, long *r) {
    int len = strlen(str);
    int endIndex = ARRAY_SIZE - (len / MAX_LEN) - 1;

    {
        int offset = 0;
        for (int i = 0; i < ARRAY_SIZE; ++i) {
            char temp[MAX_LEN + 1] = { 0 };
            int readLen = ((len - offset) < MAX_LEN) ? len - offset : MAX_LEN;
            char* start = str + len - offset - readLen;

            strncpy(temp, start, readLen);
            r[ARRAY_SIZE - i- 1] = (long)atoi(temp);
            offset += readLen;
        }
    }
}

int main() {
    const char OVER[] = "overflow\n";
    int n = 0;
    scanf("%d", &n);

    for (int i = 0; i < n; ++i) {
        scanf("%s\n%s", &inputA, &inputB);
        long a[ARRAY_SIZE] = { 0 };
        long b[ARRAY_SIZE] = { 0 };
        long r[12] = { 0 };
        int lenA = 0;
        int lenB = 0;
        const int MAX_INPUT_LEN = 80;
        lenA = strlen(&inputA[0]);
        lenB = strlen(&inputB[0]);
        if ((MAX_INPUT_LEN <= lenA) || (MAX_INPUT_LEN <= lenB)) {
            printf(OVER);
            continue;
        }

        toBigInt(inputA, a);
        toBigInt(inputB, b);
        for (int i = 0; i < ARRAY_SIZE; ++i) {
            int curRIndex = 12 - i - 1;
            r[curRIndex] += a[ARRAY_SIZE - i - 1] + b[ARRAY_SIZE - i - 1];
            const int MAX = 99999999;
            if (MAX < r[curRIndex]) {
                r[curRIndex - 1] = r[curRIndex] / (MAX + 1);
                r[curRIndex] %= (MAX + 1);
            }
        }

        if ((9999999 <= r[2]) || (0 < r[1])) {
            printf(OVER);
            continue;
        }

        {
            bool started = false;
            bool noOutput = true;
            for (int i = 0; i < 12; ++i) {
                if (!started && (0 < r[i])) {
                    started = true;
                    printf("%d", r[i]);
                    noOutput = false;
                    continue;
                }

                if (started) {
                    long temp = (r[i] == 0) ? 1 : r[i];
                    bool didFor = false;
                    for (int j = 0; temp * pow(10, j) < 1000000; ++j) {
                        printf("0");
                        didFor = true;
                    }
                    if (didFor) {
                        printf("0");
                    }
                    printf("%d", r[i]);
                }
            }

            if (noOutput) {
                printf("0");
            }
        }
        printf("\n");
        for (int i = 0; i < MAX_INPUT; ++i) {
            inputA[i] = 0;
            inputB[i] = 0;
        }
    }
    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[100],se[100],ans[100];

	scanf("%d",&N);

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

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

		
		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");
		if(keta-1<=80){
			for(j=keta-1;j>=0;j--){
				printf("%c",ans[j]);
			}
		}
		else printf("overflow");
		printf("\n");
	}

	return 0;
}

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



void add(char*a, char*b, char*c)  //鮟倩ョ、a豈巴髟ソ
{
    fill(c,c+100,0);      //蜥檎噪蜷?ス咲スョ髮カ
    int la=strlen(a), lb=strlen(b);
    int i,j;
    for( i=la,j=lb;i>la-lb&&j>0;i--,j--) // 菫晉蕗c豈泌刈謨ー譛?柄荳イ螟壻ク?ス?    {
            c[i]+=a[i-1]-'0'+b[j-1]-'0';
            if(c[i]>9)
            {
                c[i]-=10;
                c[i-1]++;
            }
    }
    for( i=la-lb;i>0;i--)
    {
            c[i]+=a[i-1]-'0';
            if(c[i]>9)
            {
                c[i]-=10;
                c[i-1]++;
            }
    }

    if(la==80&&c[0]==1)
        printf("overflow\n");
    else if(c[0]==0)
            {
                for( i=1;i<=la;i++)
                    printf("%d",c[i]);
                printf("\n");
            }
            else
            {
                for( i=0;i<=la;i++)
                   printf("%d",c[i]);
                printf("\n");
            }

}

int c[100];
int main()
{
    char a[100], b[100];
    int n;
    scanf("%d",&n);

    while(n--){
    scanf("%s%s",a,b);
    int la = strlen(a), lb = strlen(b);
    if(la>80 || lb>80)
        printf("overflow\n");
    else if(la>lb)
        add(a,b,c);
    else add(b,a,c);
    }
    return 0;
}

C++: #include<iostream>
#include<cstdio>
#include<vector>
#include<stack>
using namespace std;

int main()
{
  int n;
  cin >> n;
  getchar();
  while( n --){
    int i;
    char s[1002] = {};
    int A[1000] = {} , B[1000] = {};
    stack<int> C;
    char a, b;
    int end;
    fgets(s, 1000, stdin);
    i = 100; int k = 0;
    while(1 ) {
      a = s[k++];
      if(a  == '\n') break;
      A[i] = a - '0';
      i--;
      if(i < 20)  break;
    }
    int j = 100;
     fgets(s, 1000, stdin);
     k = 0;
    while(1) {
      b = s[k++];
      if(b == '\n') break;
      B[j] = b - '0';
      j--;
      if(j < 20)  break;

    }
    if( i < 20  || j < 20 ) {  cout << "overflow" <<endl; continue; }

    if(i > j)
      end = 101 - j;
    else
      end = 101 - i;

    A[i++] += B[j++];
  
    for(int k = 0; k <= end ; i++, j++, k++) {
      A[i] += B[j];
      A[i] += A[i - 1] / 10;
      A[i - 1] %= 10;
      C.push(A[i - 1]);
    }
    if(C.top() == 0 && C.size() != 1) {
      C.pop();
    }
    end = C.size() - 1;
    if(end > 80){
      cout << "overflow" << endl;
      continue;
    }

    for(i = 0; i < end; i++) {
      cout << C.top();
      C.pop();
    }
  
    cout << endl;
  
  }
}