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

class BigNum {
public:
	BigNum() {};

	BigNum(int n) {
		size = n;
		c = new char [size];
		memset(c, 0, size);
	};

	~BigNum() {
		if (c) {
			delete [] c;
		}
	};

	int getnum(const char* buf) {
		keta = strlen(buf);
		if (buf[keta-1] == '\n') {
			keta --;
		}
		if (keta == 0) {
			return -1;
		}
		if (c) {
			delete [] c;
		}
		size = keta;
		c = new char [size];
		int k = 0;
		for (int i = keta - 1; i >= 0; i--) {
			c[k++] = buf[i] - '0';
		}

		return 0;
	};

	void print(void) const {
		for (int i = keta - 1; i >= 0; i --) {
			printf("%d", c[i]);
		}
		printf("\n");
	};

	static int add(BigNum& x, BigNum& y) {
		BigNum* a;
		BigNum* b;

		if (x.keta > y.keta) {
			a = &x;
			b = &y;
		} else {
			a = &y;
			b = &x;
		}

		BigNum r(a->keta + 1);

		int h;
		int m = 0;
		for (int i = 0; i < a->keta; i++) {
			if (i < b->keta) {
				h = a->c[i] + b->c[i] + m;
			} else {
				h = a->c[i] + m;
			}

			m = h / 10;
			r.c[i] = h % 10;
		}

		r.keta = a->keta;

		if (m > 0) {
			r.c[r.keta] = m;
			r.keta ++;
		}

		if (r.keta > 80) {
			return 1;
		}

		r.print();

		return 0;
	};

	char* c = nullptr;
	int keta = 0;

private:
	size_t size = 0;
};

int
main(void)
{
	char line[100];
	int n;

	if (fgets(line, sizeof line, stdin) == NULL) {
		return 1;
	}
	if (sscanf(line, "%d", &n) != 1) {
		return 1;
	}

	for (int i = 0; i < n; i++) {
		if (fgets(line, sizeof line, stdin) == NULL) {
			return 1;
		}
		BigNum a;
		if (a.getnum(line)) {
			return 1;
		}

		if (fgets(line, sizeof line, stdin) == NULL) {
			return 1;
		}
		BigNum b;
		if (b.getnum(line)) {
			return 1;
		}

		if (BigNum::add(a, b)) {
			printf("overflow\n");
		}
	}

	return 0;
}

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

int main() {
	int dsn; scanf("%d", &dsn);
	for(int dsi = 0; dsi < dsn; dsi++) {
		char ais[90],bis[90];
		scanf("%s%s",ais,bis);
		int aislen = strlen(ais);
		int bislen = strlen(bis);
		int ad[90],bd[90];
		fill(ad,ad+90,0);
		fill(bd,bd+90,0);
		for(int i = 0; i < aislen; i++) ad[i] = ais[aislen-1-i]-'0';
		for(int i = 0; i < bislen; i++) bd[i] = bis[bislen-1-i]-'0';
		int cl = 0;
		for(int i = 0; i < 90; i++) {
			ad[i] += bd[i]+cl;
			cl = ad[i]/10;
			ad[i] %= 10;
		}
		bool flag = false;
		for(int i = 89; i >= 0; i--) {
			flag = flag || ad[i];
			if(flag) {
				printf("%d",ad[i]);
			}
		}
		if(!flag)printf("0");
		printf("\n");
	}
	return 0;
}

C++: #include<string>
#include<iostream>
#include<algorithm>
using namespace std;
string a,b,sum;
void check(int z,int x);
int A,B;
int main(void){
    int n;
    cin>>n;
    while(n--){
        a="",b="",sum="";
        cin>>a>>b;
        A=a.length(),B=b.length();
        reverse(a.begin(),a.end()),reverse(b.begin(),b.end());
        if(A<B)swap(A,B),swap(a,b);
        for(int i=0;i<=A-B;i++)b+="0";
        check(0,0);
        if(sum.length()>80){
            sum="overflow";
            goto end;
        }
        reverse(sum.begin(),sum.end());
        end:;
        cout<<sum<<endl;
    }
    return 0;
}
void check(int z,int x){
    int i;
    if(a[z]==0)a[z]='0';
    if(b[z]==0)b[z]='0';
    if(z==81||(z>A&&z>B&&x==1)||(z>=A&&z>=B&&x==0)){
            x=0;
            return;
    }
    i=(int)a[z]-'0' + (int)b[z]-'0' + x,x=0;
    if(i>=10)x=1,i-=10;
    sum+=i+'0';
    check(z+1,x);
}

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

int main(){
	int n;
	cin >> n;
	while( n-- > 0 ){
		string num1, num2;
		cin >> num1 >> num2;
		char sum[101];
		int degit = 0;
		for( int i = 0; i < 101; i++ ) sum[i] = '0';
		for( int i = 0; i < num1.size(); i++ ){
			sum[100-i] = num1[num1.size()-i-1];
		}
		int d;
		for( int i = 0; i < num1.size() || i < num2.size() || degit != 0; i++ ){
			int now;
			/*if( i < num2.size() ){
				now = (int)(sum[100-i]-'0') + (int)(num2[num2.size()-i-1]-'0') + degit;
			}else{
				now = (int)(sum[100-i]-'0') + degit;
			}
			*/
			sum[100-i] = (char)(now%10)+'0';
			if( now >= 10 ) degit = 1;
			else degit = 0;
			d = i;
		}
		if( d < 80 ){
			for( int i = 100-d; i < 101; i++ ) cout << sum[i];
			cout << endl;
		}else{
			cout << "overflow" << endl;
		}
	}
}

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");
		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>
#include <vector>
#include <algorithm>
#include <cmath>
#include <cstdio>
#include <functional>
#include <numeric>
#include <stack>
#include <queue>
#include <map>
#include <set>
#include <utility>
#include <sstream>
#include <complex>

using namespace std;

#define FOR(i,a,b) for(long long i=(a);i<(b);i++)
#define REP(i,N) for(long long i=0;i<(N);i++)
#define ALL(s) (s).begin(),(s).end()
#define fi first
#define se second

#define PI acos(-1.0)
#define INF 1000000007

typedef long long ll;
typedef pair<ll, ll> P;
typedef pair<string, ll> PS;
typedef vector<ll> V;

int k = 0, n;
string s, t;
vector<string> ans;

int main(){
	cin >> n;
	REP(i,n){
		cin >> s >> t;
		k = 0;
		if (s.size() >= 80 || t.size() >= 80){
			cout << "overflow" << endl;
		}
		reverse(ALL(s));
		reverse(ALL(t));
		int a = max(s.size(), t.size()), b = min(s.size(), t.size());
		if (s.size() > t.size()){
			REP(i, a - b){
				t += "0";
			}
		}
		else{
			REP(i, a - b){
				s += "0";
			}
		}		
		reverse(ALL(s));
		reverse(ALL(t));
		//cout << s << endl;
		//cout << t << endl;
		REP(i, s.size()){
			int x = (s[s.size()-1-i] - '0') + (t[s.size()-1-i] - '0');
			string q;
			stringstream ss;
			if (i != s.size() - 1){
				ss << ((x + k) % 10);
			}
			else{
				ss << x + k;
			}
			ss >> q;
			ans.push_back(q);
			if (x + k >= 10){
				k = (x + k) / 10;
			}
			else{
				k = 0;
			}
		}
		reverse(ALL(ans));
		REP(i, ans.size()){
			cout << ans[i];
		}
		cout << endl;
		ans.clear();
	}
}