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<bits/stdc++.h>
using namespace std;
int main()
{
    int n;
    while(~scanf("%d",&n))
    {
        int prime[n+1];
        for(int i=0;i<=n;i++)
            prime[i]=1;
            prime[0]=0;
            prime[1]=0;
        for(int i=2;i*i<=n;i++)
             if(prime[i]==1)
        {
            for(int j=2;j*i<=n;j++)
            prime[i*j]=0;
        }
        int c=0;
        for(int i=0;i<=n;i++)
        {
            if(prime[i]==1)
                c++;
        }
        printf("%d\n",c);
    }

}

C++: #define _CRT_SECURE_NO_WARNINGS

#include <stdio.h>
#include <string.h>

using namespace std;

const size_t opsize = 81, bufsize = 102;
unsigned char op1[opsize], op2[opsize];
char buf[bufsize];

bool readOp(unsigned char * op){
    memset(op, 0, opsize);
    
    fgets(buf, bufsize, stdin);
    int l = strlen(buf);
    if(l > 81) return true;

    char *p = &buf[l-2];
    for (int j = 0; j < l-1; j++) {
        op[j] = *p - '0';
        p--;
    }
    return false;
}

void printOp(unsigned char * op){
    int mh = opsize - 1;
    while(op[mh] == 0 && mh > 0) mh--;
    for(; mh >= 0; mh--){
        putchar(op[mh] + '0');
    }
    putchar('\n');
}

void plus(){
    for (int i = 0; i < opsize - 1; i++) {
        op1[i] += op2[i];
        if(op1[i] >= 10){
            op1[i] -= 10;
            op1[i+1] += 1;
        }
    }
}

int main(int argc, char const* argv[])
{
    int n;
    scanf("%d ", &n);
    for (int i = 0; i < n; i++) {
        bool overflow = false;
        overflow |= readOp(op1);
        overflow |= readOp(op2);
        if(overflow){
            puts("overflow");
            continue;
        }
        plus();
        printOp(op1);
    }
    
    return 0;
}

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

using namespace std;

int main()
{
	string str1, str2, ans;
	int n;

	cin >> n;

	for (int i=0; i<n; i++)
	{
		bool cryflag = false,	//trueのとき繰り上がり
		     overflag = false;	//trueのときoverflow
		ans.clear();
		cin >> str1 >> str2;

		int len1 = str1.size(), len2 = str2.size();

		for (int j=0, k=0; j<len1 || k<len2; j++, k++) {
			int c1 = 0, c2 = 0, c;

			if(j < len1)
				c1 = str1[len1-j-1] - '0';
			if(k < len2)
				c2 = str2[len2-k-1] - '0';

			//cout << len1 << len2;	//Debug

			c = c1 + c2;

			if(cryflag) {
				c++;
				cryflag = false;
			}

			if(c>=10)	cryflag = true;

			ans += c%10 + '0';

			//cout << len1 << len2;	//Debug

			if(ans.size() > 80 || (ans.size() == 80 && cryflag))	{
				overflag = true;
				break;
			}
		}

		if(overflag) {
			cout << "overflow";
		}
		else {
			if(cryflag)	cout << "1";
			for (int i=0; i<(int)ans.size(); i++) {
				cout << ans[ans.size()-i-1];
			}
		}

		cout << endl;
	}

	return 0;
}

C++: #include <cstdio>
#include <iostream>

using namespace std;

int main(void) {
	int sets;
	char input1[100], input2[100], out[100];
	int i1, i2, o, carry;
	int ic1, ic2;
	int i;

	cin >> sets;
	for(i=0;i<sets;i++) {
		scanf("%s", input1);
		scanf("%s", input2);
		for(i1=0;input1[i1] != '\0';i1++);
		for(i2=0;input2[i2] != '\0';i2++);
		i1--;i2--;o=0;carry=0;
		while(i1>=0 || i2>=0 || carry) {
			if(o > 80) { break; }
			if(i1>=0) { ic1 = input1[i1]-'0'; i1--; } else { ic1 = 0; }
			if(i2>=0) { ic2 = input2[i2]-'0'; i2--; } else { ic2 = 0; }
			if( ic1+ic2+carry < 10 ) { 
				out[o] = ic1+ic2+carry+'0';
				carry = 0;
			} else {
				out[o] = ic1+ic2+carry-10+'0';
				carry = 1;
			}
			o++;
		}
		if(o > 80) { printf("overflow"); }
		else { for(o--;o>=0;o--) { printf("%c", out[o]); } }
		printf("\n"); 
	}

	return 0;
}

C++:  #include<iostream>
 #include<stdio.h>
 #include<string>
 #include<math.h>
 #include<iomanip>
 #include<algorithm>
 #include<string.h>
 #include<cctype>
 #include<map>
 #include<set>
 #include<vector>
 #include<sstream>
 #include<stack>
 
 using namespace std;
 
 int main()
 {
   int N;
   cin>>N;
   while(N--)
   {
     string a,b,sum;
     cin>>a>>b;
     bool flag=false;
     if(a.length()>=80||b.length()>=80) flag=true;
     else
     {
       int up=0;
       int i;
       for(int j=0;j<a.length()/2;j++) swap(a[j],a[a.length()-j-1]);
       for(int j=0;j<b.length()/2;j++) swap(b[j],b[b.length()-j-1]);
       for(i=0;i<a.length()&&i<b.length();i++)
       {
         sum+=(((a[i]-'0')+(b[i]-'0')+up)%10)+'0'; 
         if(a[i]-'0'+b[i]-'0'+up>=10) up=1;   
         else up=0;  
       }          
       while(i<a.length()) 
       {
         sum+=((a[i]-'0'+up)%10)+'0';
         if(a[i]-'0'+up>=10) up=1; 
         else up=0;         
         i++;          
       }
       while(i<b.length()) 
       {
         sum+=((b[i]-'0'+up)%10)+'0';
         if(b[i]-'0'+up>=10) up=1; 
         else up=0;
         i++;                   
       }
       if(up==1) sum+='1';
       if(sum.length()>=80) flag=true;
     }
     if(flag) cout<<"overflow"<<endl;
     else
     {
       for(int i=sum.length()-1;i>=0;i--) cout<<sum[i];
       cout<<endl;    
     }
   }
   //while(1);      
   return 0;    
 }

C++: 
#define _USE_MATH_DEFINES
#include <cfloat>
#include <climits>
#include <cmath>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <ctime>
#include <iostream>
#include <memory>
#include <string>

#include <algorithm>
#include <complex>
#include <list>
#include <map>
#include <numeric>
#include <queue>
#include <set>
#include <stack>
#include <vector>

using namespace std;

#ifdef _MSC_VER
#define _CRT_SECURE_NO_WARNINGS
#define __typeof__ decltype
#endif

#define ITER(c)         __typeof__((c).begin())
#define FOREACH(it, c)  for (ITER(c) it=(c).begin(); it != (c).end(); ++it)
#define RITER(c)        __typeof__((c).rbegin())
#define RFOREACH(it, c) for (RITER(c) it=(c).rbegin(); it != (c).rend(); ++it)
#define REP(i, n)       REPEAT(i, 0, n)
#define REPEAT(i, k, n) for (int i = (k); i < (n); ++i)
#define REVREP(i, n)    for (int i = ((n)-1); i >= 0; --i)
#define ESIZEOF(A)      (sizeof(A)/sizeof((A)[0]))
#define MIN(p1,p2)      (((p1)<(p2))?(p1):(p2))
#define MAX(p1,p2)      (((p1)>(p2))?(p1):(p2))
#define CIN_NO_SYNC     do { cin.tie(0); ios_base::sync_with_stdio(false); } while(0)

bool add(char *d, int n, string &s){
    int l = s.length();
    int j = 0;
    int carry = 0;
    while(carry || j < l){
        if(j >= n) return true;
        int dgt = (j < l) ? (s[l-j-1] - '0') : 0;
        d[j] = d[j] + dgt + carry;
        carry = d[j] / 10;
        d[j] %= 10;
        j++;
    }
    return false;
}
void print(const char *d, int n){
    bool pf = false;
    for(int i = n-1; i >= 0; i--){
        if(d[i] != 0 || pf || i == 0){
            pf = true;
            printf("%c", '0' + d[i]);
        }
    }
    printf("\n");
}

int main(void){
    CIN_NO_SYNC;
    int n;
    cin >> n;
    char d[80];

    REP(i,n){
        string s1, s2;
        cin >> s1 >> s2;
        //cout << s1 << ":" << s2 << endl;
        memset(d, 0, sizeof(d));
        if(add(d, sizeof(d), s1) || add(d, sizeof(d), s2)){
            printf("overflow\n");
        } else {
            print(d, sizeof(d));
        }
    }
    return 0;
}