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 <cstdio>
#include <cmath>
#include <cstdlib>
#include <cstring>

using namespace std;

const int MAX_INPUT = 1000000;
const int ARRAY_SIZE = 11;
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]) {
            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];
                    for (int j = 0; temp * pow(10, j) < 10000000; ++j) {
                        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
//------------------------------------------
#include <bits/stdc++.h>

using namespace std;

////typedef
////------------------------------------------
//typedef vector<int> vi;
//typedef vector<vi> vii;
//typedef vector<string> vs;
//typedef vector<bool> vb;
//typedef vector<vb> vbb;
//typedef pair<int, int> pii;
//typedef long long ll;
//typedef unsigned long long ull;
//
////container util
////------------------------------------------
//#define all(a)  (a).begin(),(a).end()
//#define rall(a) (a).rbegin(), (a).rend()
//#define pb push_back
//#define mp make_pair
//#define iter(i,c) for(auto i=(c).begin(); i!=(c).end(); ++i)
//
////repetition
////------------------------------------------
//#define loop(i,a,b) for(int i=(a);i<(b);++i)
//#define rep(i,n)  loop(i,0,n)
//
////constant
////--------------------------------------------
//const double eps = 1e-10;
//const double pi  = acos(-1.0);
//const double inf = (int)1e8;
//
//// output vector
//// ------------------------------------------
//namespace std {
//    template<typename T> ostream & operator<<(ostream& os, vector<T> const& v){
//        for(auto it=v.begin(); it!=v.end(); ++it){
//            os << *it << (it+1==v.end() ? "" : ",");
//        }
//        return os;
//    }
//}
//
////clear memory
//#define clr(a) memset((a), 0 ,sizeof(a))

string IntToString(int number) {
    stringstream ss;
    ss << number;
    return ss.str();
}

int main(){
    int n;
    cin >> n;
    for(int p=0; p < n; p++){
        string ans;
        string s, t;
        cin >> s >> t;
        if(s.size() > t.size()) swap(s, t);


        int q = t.size()-s.size();
        for(int i=0; i < q; i++) s = "0" + s;

        int up = 0;
        for(int i=s.size()-1; 0 <= i; i--){
            int d = atoi(s.substr(i, 1).c_str());
            int f = atoi(t.substr(i, 1).c_str());
            ans = IntToString((d+f+up)%10) + ans;

            if(0 <= d+f+up-10) up = 1;
            else up = 0;
        }
        if(up == 1) ans = IntToString(1) + ans;
        cout << ans << endl;
     }
}

C++: #include <cstdio>
#include <cstdlib>
#include <iostream>
#include <string>
#include <cmath>
#include <algorithm>
#include <vector>
#include <queue>
#include <stack>

using namespace std;
typedef long long ll;

#define PI 3.141592653

int main(){
    string A,B;
    int n,Ak,Bk,ans[81];
    
    scanf("%d",&n);
    
    for(int i=0;i<n;i++){
        
        for(int j=0;j<80;j++){
            ans[j]=0;
        }
        
        cin >> A >> B;
        
        if(A==B && A=="0"){
            printf("0\n");
            continue;
        }
        
        Ak=(int)A.size();
        Bk=(int)B.size();
        
        if(Ak>80 || Bk>80){
            puts("overflow");
            continue;
        }
        
        for(int j=0;j<max(Ak,Bk);j++){
            if(j<min(Ak,Bk))
                ans[79-j]+=(A[Ak-1-j]-'0')+(B[Bk-1-j]-'0');
            else if(j<Ak)
                ans[79-j]+=A[Ak-1-j]-'0';
            else if(j<Bk)
                ans[79-j]+=B[Bk-1-j]-'0';
            
            if(ans[79-j]>=10){
                if(j==79){
                    break;
                }
                ans[79-j-1]++;
                ans[79-j]-=10;
            }
        }
        
        if(ans[79]>=10){
            puts("overflow");
        }else{
            bool f = false;
            for(int j=0;j<80;j++){
                if(ans[j]>0 || f){
                    printf("%1d",ans[j]);
                    f = true;
                }
            }
            printf("\n");
        }
    }
    
    return 0;
}