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<algorithm>
using namespace std;
const int INF=10000;
char a[1001],b[1001],c[1001];
void rev(int pnt,char *cp){
    for(int i=0;i<pnt/2;i++){
        char work;
        work=*(cp+i);
        *(cp+i)=*(cp+pnt-i-1);
        *(cp+pnt-i-1)=work;
    }
}
int main(){
    int n;
    cin>>n;
    while(n--){
        for(int i=0;i<1000;i++)a[i]=b[i]=c[i]=0;
        cin>>a>>b;

        int apnt,bpnt;
        for(int i=0;i<1000;i++)if(a[i]==0){apnt=i;break;}
        for(int i=0;i<1000;i++)if(b[i]==0){bpnt=i;break;}

        if(apnt>80 || bpnt>80){cout<<"overflow"<<endl;continue;}

        rev(apnt,a);
        rev(bpnt,b);
        int d[100];
        for(int i=0;i<100;i++)d[i]=0;
        int big=max(apnt,bpnt);
        for(int i=0;i<big;i++){
            if(i<apnt)d[i]+=a[i]-'0';
            if(i<bpnt)d[i]+=b[i]-'0';
        }
        for(int i=0;i<big;i++){
            if(d[i]>=10){
                d[i]%=10;
                d[i+1]++;
            }
        }
        if(d[big]==1)big++;
        if(big>80)continue;

        for(int i=0;i<big;i++){
            c[i]=d[i]+'0';
        }
        rev(big,c);
        cout<<c<<endl;
    }
    return 0;
}

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)((char)(now%10)+'0');
			if( now >= 10 ) degit = 1;
			else degit = 0;
			d = i;
		}
		for( int i = 100-d; i < 101; i++ ) cout << sum[i];
		cout << endl;
	}
}

C++: #include <stdio.h>
#include <string.h>
#define N 80

int main(void){
    char buf[N+1];
    int datas;
    int degits;
    int lint1[N+1], lint2[N+1], sum[N+1];
    int cry;
    
    scanf("%d", &datas);
    for (int n=0; n<datas; n++){
        // reset
        for (int i=0; i<N; i++) lint1[i]=lint2[i]=0;
        // input
        scanf("%s", buf);
        degits = strlen(buf);
        for (int i=0; i<degits; i++){
            lint1[degits-i-1]= buf[i]-'0';
        }
        scanf("%s", buf);
        degits = strlen(buf);
        for (int i=0; i<degits; i++){
            lint2[degits-i-1]= buf[i]-'0';
        }
        // calc
 /*       cry=0;
        for (int i=0; i<N; i++){
            sum[i] = lint1[i]+lint2[i] + cry;
            cry = sum[i]>9 ? 1:0;
            if (cry) sum[i] -= 10;
        }
   */     // output
        if (cry){
            puts("overflow");
        } else {
            bool zero=true;
            if (cry) puts("1");
            for (int i=N-1; i>=0; i--){
                if (zero && sum[i] != 0) zero=false;
                if (!zero) printf("%d", sum[i], i);
            }
            printf("\n");
        }
        
    }
    
    return 0;
}

C++: #include <iostream>
#include <cstring>
#define N 10000
using namespace std;
void mpstr2num(int *num, char *str);
void mpadd(int* ret, int* a, int* b);
void mpnum2str(char* str, int* num);

char a[100], b[100];
int anum[25], bnum[25];
int resultnum[25];
char result[100];

void mpstr2num(int* num, char* str){
    char* ss;
    int* nn;
    while(*str == ' ' || *str == '\t') str++;
    while(*str == '0') str++;
    ss = str;
    while(*ss >= '0' && *ss <= '9') ss++;
    if(ss == str){
        *num = 0;
        return;
    }
    int x = 0;
    int k = 1;
    nn = num;
    do{
        x+=(*--ss - '0') * k;
        k *= 10;
        if(k == N || ss == str){
            *++nn = x;
            x = 0;
            k = 1;
        }
    }while(ss != str);

    *num = nn - num;
    return;
}

void mpadd(int* ret, int* a, int* b){
    int la = *a;
    int lb = *b;
    int lr = (la >= lb) ? la : lb;
    int* rr = ret;
    int x = 0;
    for(int i=1; i<= lr; i++){
        if(i <= la) x += *++a;
        if(i <= lb) x += *++b;
        if(x < N){
            *++rr = x;
            x = 0;
        }else{
            *++rr = x - N;
            x = 1;
        }
    }
    *++rr = x;
    *ret = lr + x;
    return;
}

void mpnum2str(char* str, int* num){
    if(*num == 0){
        *str++ = '0';
        *str = '\0';
        return;
    }
    char* ss = str - 1;
    int x;
    for(int i=*num; i>0; i--){
        x = *++num;
        for(int j=1; j<N; j*=10){
            *++ss = x%10 + '0';
            x /= 10;
        }
    }    while(*ss == '0') ss--;
    *(ss+1) = '\0';
    while(str < ss){
        x = *str;
        *str++ = *ss;
        *ss-- = x;
    }
    return;
}

int main(){
    int n;
    cin >> n;
    for(int i=1; i<=n; i++){
        cin >> a >> b;
        if(strlen(a) > 80 || strlen(b) > 80){
            cout << "overflow" << endl;
            continue;
        }
        mpstr2num(anum, a);
        mpstr2num(bnum, b);
        mpadd(resultnum, anum, bnum);
        mpnum2str(result, resultnum);
        if(strlen(result) > 80){
            cout << "overflow" << endl;
            continue;
        }else{
            cout << result << endl;
        }
    }
    return 0;
}

C++: #include <iostream>
#include <sstream>
#include <cstdio>
#include <cstdlib>
#include <cmath>
#include <ctime>
#include <cstring>
#include <climits>
#include <algorithm>
#include <map>
#include <set>
#include <stack>
#include <vector>
#include <queue>
#include <string>
#include <complex>
using namespace std;

#define rep(i,n) for(int i=0; i<n; i++)
#define repa(i,s,e) for(int i=s; i<=e; i++)
#define repd(i,s,e) for(int i=s; i>=e; i--)

typedef vector<int> vi;

int n;
string s1, s2;
char n1[81], n2[81], sum[81];

int main() {
	scanf("%d", &n);
	rep(i,n) {
		memset(n1, 0, sizeof(n1));
		memset(n2, 0, sizeof(n2));
		memset(sum, 0, sizeof(sum));

		cin>>s1>>s2;
		int l1 = s1.size();
		int l2 = s2.size();
		rep(k,l1) n1[k] = s1[l1-k-1] - '0';
		rep(k,l2) n2[k] = s2[l2-k-1] - '0';

		char a = 0;
		rep(i,80) {
			char c = n1[i] + n2[i] + a;
			a = c / 10;
			sum[i] = c % 10;
		}

		bool start = false;
		repd(i,80,0) {
			if(sum[i] != 0) start = true;
			if(!start && sum[i] == 0) continue;
			printf("%d", sum[i]);
		}
		printf("\n");
	}
}

C++: #include <bits/stdc++.h>
using namespace std;

int main(void){
	string s[55][2];
	int ans[100],k,i,p,q;
	bool flag;
	cin >> q;
	for(p=0; p<q; p++){
		cin >> s[p][0];
		cin >> s[p][1];
		fill(ans,ans+100,0);
		if( s[p][0].length()>80 || s[p][1].length()>80 ){
			cout << "overflow" << endl;
			continue;
		}
		for(k=0; k<2; k++){
			reverse(s[p][k].begin(),s[p][k].end());
			for(i=(s[p][k].length()-1); i>=0; i--){
				ans[i] += (s[p][k][i] - '0');
			}
		}
		for(i=0; i<80; i++){
			ans[i+1] += (ans[i]/10);
			ans[i] %= 10;
		}
		flag = true;
		for(i=99; i>79; i--){
			if(ans[i] != 0){
				cout << "overflow" << endl;
				flag = false;
				break;
			}
		}
		if(!flag){
			continue;
		}
		flag = false;
		for(i=79; i>=0; i--){
			if(flag){
				cout << ans[i];
			}else if(ans[i]!=0){
				cout << ans[i];
				flag = true;
			}
		}
		cout << endl;
	}
	return 0;
}