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 <vector>
#include <iostream>
#include <set>
#include <cstdio>
#include <queue>
#include <cstdio>
#include <cstdlib>
#include <cassert>
#include <ctime>
#include <cmath>
#include <complex>
#include <algorithm>
#include <tuple>
#include <algorithm>
#include <limits>
#include <map>
#include <valarray>
#include <list>

using namespace std;

typedef long long ll;

int main() {
    int t;
    cin >> t;
    while (t--) {
        string s1, s2;
        cin >> s1 >> s2;
        if (s1 == "0" && s2 == "0") {
            cout << "0" << endl;
            continue;
        }
        reverse(s1.begin(), s1.end());
        reverse(s2.begin(), s2.end());
        int n = max(s1.size(), s2.size()) + 1;
        for (int i = s1.size(); i < n; i++) {
            s1 += '0';
        }
        for (int i = s2.size(); i < n; i++) {
            s2 += '0';
        }
        string res;
        int b = 0;
        for (int i = 0; i < max(s1.size(), s2.size()); i++) {
            int u = s1[i]-'0'+s2[i]-'0'+ b;
            b = (u >= 10); u %= 10;
            res += u + '0';
        }
        reverse(res.begin(), res.end());
        string res2;
        int j = 0;
        while (res[j] == '0') j++;
        for (; j < res.size(); j++) {
            res2 += res[j];
        }
        if (res2.size() > 80) {
            cout << "overflow" << endl;
        } else {
            cout << res2 << endl;
        }
    }
}

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

int main()
{
    ios::sync_with_stdio(false);
    cin.tie(0);
    int n;
    cin>>n;
    cin.ignore();
    for(int i=0; i<n; i++){
        char a[128], b[128], c[128];
        int al, bl;
        cin.getline(a, 128);
        al=cin.gcount()-1;
        cin.getline(b, 128);
        bl=cin.gcount()-1;
        int j=al, k=bl, carry=0, l=0;
        while(true){
            j--, k--;
            if(j<0||k<0) break;
            c[l]=a[j]+b[k]+carry-'0';
            if(c[l]>'9'){
                c[l]-=10;
                carry=1;
            } else {
                carry=0;
            }
            ++l;
        }

        if(j<0){
            ++k;
            while(--k>=0){
                c[l]=b[k]+carry;
                if(c[l]>'9'){
                    c[l]-=10;
                    carry=1;
                } else {
                    carry=0;
                }
                ++l;
            }
        }else{
            ++j;
            while(--j>=0){
                c[l]=a[j]+carry;
                if(c[l]>'9'){
                    c[l]-=10;
                    carry=1;
                } else {
                    carry=0;
                }
                ++l;
            }
        }
        if(carry==1) c[l++]='1';
        if(l>80) cout<<"overflow\n";
        else{
            while(--l>=0) cout<<c[l];
            cout<<'\n';
        }
    }
    return 0;
}

C++: #include <stdio.h>
#include <stdlib.h>
#include <string>
#include <iostream>
#include <math.h>
#include <stack>

using namespace std;

int main(){
    
    char d;
    char e;
    using namespace std;
    stack<int> st1;
    stack<int> st2;
    stack<int> st3;
    int n;
    //scanf("%d",&n);
    while((d=getchar())!='\n'){
        n=atof(&d);
    }
    
    for(int i=0; i<n; i++){
        
        while((d=getchar())!='\n'){
            //printf("a%c\n",d);
            st1.push(atof(&d));
        }
        
        //while(getchar()!='\n'){}
        
        while((e=getchar())!='\n'){
            //printf("b%c\n",e);
            st2.push(atof(&e));
        }
        int temp=0;
        int num=0;
        
        while(st1.empty() != true || st2.empty() != true){
            if(st1.empty()==true && st2.empty()!=true){
                temp+=st2.top();
                st2.pop();
            }else if(st1.empty()!=true && st2.empty()==true){
                temp+=st1.top();
                st1.pop();
            }else{
                temp+=st1.top()+st2.top();
                st1.pop();
                st2.pop();
            }
            st3.push(temp%10);
            temp=temp/10;
            num++;
        }
        //if(temp!=0){
            st3.push(temp);
        //}
        if(num>80){
            printf("overflow");
        }else{
            while(st3.empty()!=true){
                cout << st3.top();
                st3.pop();
            }
        }
        printf("\n");
        
        
        
    }
    
}

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

int main(void){
	string a,b;
	vector<int> ans;
	int n,num,ai,bi,c=0;
	bool flag;

	cin >> n;

	if(n>0&&n<=100){
		for(int i=0;i<n;i++){
			cin >> a >> b;
			flag=false;	

			for(int j=0;j<100;j++){
				num=0;
				ai=a.size()-j-1;
				bi=b.size()-j-1;
				if(ai<0&&bi<0&&c==0) break;
				if(ai>=0) num+=a[ai]-48;
				if(bi>=0) num+=b[bi]-48;
				ans.push_back((num+c)%10);
				if(num+c>=10) c=1;
				else c=0;
			}
			if(ans.size()>80){
				flag=true;
			}
			if(flag){
				cout << "overflow";
			}else{
				for(int j=ans.size()-1;j>=0;--j){
					cout << ans[j];
				}
				cout << endl;
				ans.clear();
			}
		}
	}
	return 0;
}

C++: #include<iostream>
#include<string>
#include<cstring>
using namespace std;
string S, T; int n, x[10000], a[10000], b[10000], cnt; char U[11] = "0123456789";
int main() {
	cin >> n;
	for (int times = 0; times < n; times++) {
		cin >> S >> T;
		memset(x, 0, sizeof(x));
		memset(a, 0, sizeof(a));
		memset(b, 0, sizeof(b));
		for (int i = 0; i < S.size(); i++) {
			for (int j = 0; j < 10; j++) {
				if (U[j] == S[i]) {
					a[S.size() - 1 - i] = j;
				}
			}
		}
		for (int i = 0; i < T.size(); i++) {
			for (int j = 0; j < 10; j++) {
				if (U[j] == T[i]) {
					b[T.size() - 1 - i] = j;
				}
			}
		}
		if (S.size() > 80 || T.size() > 80) {
			cout << "OVERFLOW" << endl; goto E;
		}
		for (int i = 0; i < 80; i++) {
			x[i] += (a[i] + b[i]) % 10;
			x[i + 1] += (a[i] + b[i]) / 10;
			if (x[i] >= 10) {
				x[i + 1] += x[i] / 10;
				x[i] %= 10;
			}
			if (x[i + 1] >= 10) {
				x[i + 2]++;
				x[i + 1] %= 10;
			}
		}
		cnt = 0;
		if (x[80] > 0) {
			cout << "OVERFLOW" << endl;
		}
		else {
			for (int i = 79; i >= 0; i--) {
				if (x[i] > 0 || i == 0) { cnt++; }
				if (cnt) { cout << x[i]; }
			}
			cout << endl;
		}
	E:;
	}
}