Go: package main

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

var scanner = bufio.NewScanner(os.Stdin)

func nextString() string {
	scanner.Scan()
	return scanner.Text()
}

func nextInt() int {
	n, err := strconv.Atoi(nextString())
	if err != nil {
		fmt.Printf("strconv.Atoi failed: %v\n", err)
	}
	return n
}

func main() {
	scanner.Split(bufio.ScanWords)

	const maxCapacity = 512 * 1024
	buf := make([]byte, maxCapacity)
	scanner.Buffer(buf, maxCapacity)

	n := nextInt()
	maxint, _ := new(big.Int).SetString("100000000000000000000000000000000000000000000000000000000000000000000000000000000", 10)

	for i := 0; i < n; i++ {
		aStr := nextString()
		bStr := nextString()

		a, _ := new(big.Int).SetString(aStr, 10)
		b, _ := new(big.Int).SetString(bStr, 10)
		c := big.NewInt(0).Add(a, b)

		if c.Cmp(maxint) < 0 {
			fmt.Println(c)
		} else {
			fmt.Println("overflow")
		}
	}
}

C++: #include <stdio.h>

#define BUF_SIZE 256

int main(void) {
	char buf[BUF_SIZE];
	int num1[81];
	int num2[81];
	int sum[81];
	char str[82];
	int n;
	int i, j, k;
	
	fgets(buf, sizeof(buf), stdin);
	sscanf(buf, "%d", &n);
	
	for (i=0; i<n; i++) {
		for (j=0; j<80; j++) {
			num1[j] = num2[j] = 0;
		}
		for (j=0; j<81; j++) {
			sum[j]=0;
		}		
		
		fgets(buf, sizeof(buf), stdin);
		sscanf(buf, "%s", &str);
		j=k=0;
		while (str[j] != '\0')  j++;
		while (--j >= 0) num1[k++] = (int)(str[j] - '0'); 
		
		fgets(buf, sizeof(buf), stdin);
		sscanf(buf, "%s", &str);
		j=k=0;
		while (str[j] != '\0')  j++;
		while (--j >= 0) num2[k++] = (int)(str[j] - '0'); 
		
		for (j=0; j<80; j++) {
			sum[j] += num1[j]+num2[j];
			if (sum[j] >= 10) {
				sum[j+1] += 1;
				sum[j] = sum[j]%10;
			}
		}
		
		if (sum[80] != 0) {
			printf("overflow\n");
		}
		else {
			j=79;
			while (sum[j] == 0)  j--;
			for (; j>=0; j--) {
				printf("%d", sum[j]);
			}
			printf("\n");
		}
	}
	
	return 0;
}

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

using namespace std;

int main(void) {
	int sets;
	char input1[500], input2[500], out[500];
	int i1, i2, o, carry;
	int ic1, ic2;
	int i;

	cin >> sets;
	for(i=0;i<sets;i++) {
		scanf("%s", input1);
		for(i1=0;input1[i1] != '\0';i1++);
		scanf("%s", input2);
		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\n");
		}
		else {
			for(o--;o>=0;o--) {
				printf("%c", out[o]);
			}
			printf("\n");
		}
	}

	return 0;
}

C++: #include<iostream>
#include<vector>
#include<string>
#include<cstdlib>
using namespace std;

int main(void){
	int n;
	cin >> n;
	for(int i=0; i<n; i++){
		string a, b;
		cin >> a >> b;
		if(a.size()>80 || b.size()>80) {cout << "overflow" << endl;}
		else{
			vector<int> x, y, z;
			for(int j=0; j<80; j++){
				x.push_back(0);
				y.push_back(0);
				z.push_back(0);
			}

			for(int j=0; j<a.size(); j++)
				x[j] = atoi( a.substr(a.size()-1-j,1).c_str() );
			for(int j=0; j<b.size(); j++)
				y[j] = atoi( b.substr(b.size()-1-j,1).c_str() );
			
			for(int j=0; j<80; j++){
				z[j] = x[j]+y[j];
			}
			for(int j=0; j<79; j++){
				if( z[j]>=10 ){
					z[j+1]++;
					z[j]-=10;
				}
			}
			if(z[79]>=10) {cout << "overflow" << endl;}
			else {
				bool zeroume = false;
				for(int j=0; j<80; j++){
					if(z[79-j]==0 && zeroume==false){}
					else{cout << z[79-j];zeroume=true;}
				}
				if(zeroume==false) cout << 0;
				cout << endl;
			}
		}
	}
	return 0;
}

C++: #include <iostream>
#include <stdio.h>
#include <sstream>
#include <string>
#include <vector>
#include <map>
#include <queue>
#include <algorithm>
#include <set>
#include <math.h>
#include <utility>
#include <stack>
using namespace std;
typedef pair<int,int> P;
const int INF = ~(1<<31) / 2;

int main(){
    int k;
    cin >> k;
    for(int j=0;j<k;j++){
        int n[80];
        string a,b;
        cin >> a >> b;
        int n_a = a.size(),n_b = b.size();
        if(n_a>80||n_b>80){cout << "overflow" << endl;continue;}

        fill(n,n+80,0);
        bool is_over = false;
        for(int i=0;i<max(n_a,n_b);i++){
            int ai,bi;
            if (i<n_a) ai = a[n_a-i-1]-'0';
            else ai = 0;
            if (i<n_b) bi = b[n_b-i-1]-'0';
            else bi = 0;
            n[i] += ai + bi;
            if (n[i]>=10){
                if(i+1 == 80){
                    is_over = true;
                    break;
                }else{
                    n[i+1] += n[i]/10;
                    n[i] = n[i]%10;
                }
            }
        }
        if (is_over){
            cout << "overflow" << endl;
        }else{
            bool started = false;
            for(int i=79;i>=0;i--){
                if(!started && n[i]>0){
                    started = true;
                }
                if(started){
                    cout << n[i];
                }
            }
            cout << endl;
        }
    }
    return 0;
}

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

using namespace std;

int A[90],B[90],C[90];

int main()
{
    int n;
    cin >> n;
    while(n > 0)
    {
        n--;
        string str1,str2;
        cin >> str1 >> str2;
        int len1 = str1.length();
        int len2 = str2.length();
        if(len1 < len2)
        {
            string st = str1;
            str1 = str2;
            str2 = st;
            int ll = len1;
            len1 = len2;
            len2 = ll;
        }
        for(int i = 0; i < 90; i++)
            C[i] = 0;
        for(int i = 0; i < len1; i++)
            A[i] = str1[len1 - 1 - i] - '0';
        for(int i = 0; i < len2; i++)
            B[i] = str2[len2 - 1 - i] - '0';
        for(int i = 0 ; i < len2; i++)
        {
            C[i] = (A[i] + B[i]) % 10;
            C[i+1] = (A[i] + B[i]) / 10;
        }
        for(int i = len2; i <len1; i++)
        {
             C[i] = C[i] + A[i];
             C[i+1] = C[i] / 10;
             C[i] = C[i] % 10;
        }


        if(C[80] > 0)
            cout << "overflow" << endl;
        else
            {
                if(C[len1] > 0)
                    len1++;
                for(int i = len1-1; i >= 0; i--)
                    cout << C[i];
                cout << endl;
            }
    }
    return 0;
}

C++: #include <iostream>
#include <string.h>
#include <algorithm>

using namespace std;
void add(string& ans,string& a,string& b);
int main() {
	string a,b;
	int al,bl;
	int n;
	cin>>n;
	while(n--){
	        string ans= "";
		cin>>a>>b;
		reverse(a.begin(),a.end());
		reverse(b.begin(),b.end());
		al=a.size();
		bl=b.size();
		if(bl>al){
			swap(a,b);
			swap(al,bl);
		}
		for(int i=0;i<(al-bl);i++){
			b+='0';
		}
		add(ans,a,b);
		if(ans.size()>80) ans="overflow";
		else{
		  reverse(ans.begin(),ans.end());
		}
	  	cout<<ans<<endl;
	}
	return 0;
}
void add(string& ans,string& a,string& b){
	int x,y=0;
	for(int i= 0;i<a.size();i++){
		x=a[i]-'0'+b[i]-'0';
		if(x>=10){
			ans+= (x%10)+'0';
			y=1;
		}else{
			ans+= x+y+'0';
			y=0;
		}
	}
	if(y==1)ans+='1';
}