Go: package main

import (
        "fmt"
        "bufio"
        "os"
)

func main() {
        var n int
        var a, b string
        sc := bufio.NewScanner(os.Stdin)
        fmt.Scanf("%d", &n)
        for i := 0; i < n; i++ {
                sc.Scan()
                a = sc.Text()
                sc.Scan()
                b = sc.Text()

                if len(a) > len(b) {
                        s := add(a, b)
                        if s[len(a)] > 0 {
                                if len(a) >= 80 {
                                        fmt.Printf("overflow")
                                } else {
                                        for i := 0; i <= len(a);i++ {
                                                fmt.Printf("%d", s[len(a)-i])
                                        }
                                }
                                fmt.Printf("\n")
                        } else {
                                if len(a)-1 >= 80 {
                                        fmt.Printf("overflow")
                                } else {
                                        for i := 0; i < len(a);i++ {
                                                fmt.Printf("%d", s[len(a)-i-1])
                                        }
                                }
                                fmt.Printf("\n")
                        }
                } else {
                        s := add(b, a)
                        if s[len(b)] > 0 {
                                if len(b) >= 80 {
                                        fmt.Printf("overflow")
                                } else {
                                        for i := 0; i <= len(b);i++ {
                                                fmt.Printf("%d", s[len(b)-i])
                                        }
                                }
                                fmt.Printf("\n")
                        } else {
                                if len(b)-1 >= 80 {
                                        fmt.Printf("overflow")
                                } else {
                                        for i := 0; i < len(b);i++ {
                                                fmt.Printf("%d", s[len(b)-i-1])
                                        }
                                }
                                fmt.Printf("\n")
                        }
                }
        }
}

// expects ln > lm
func add(n string, m string) (s [100]int) {
        ln := len(n)
        lm := len(m)
        var sum [100]int

        for i := 0; i < ln; i++ {
                var s, t int
                s = int(n[ln-1-i]) - int('0')

                if lm - 1 - i >= 0 {
                        t = int(m[lm-1-i]) - int('0')
                } else {
                        t = 0
                }

                sum[i+1] = (s + t + sum[i]) / 10
                sum[i] = (s + t + sum[i]) % 10
        }

        return sum
}

C++: #include <string>
#include <vector>
#include <algorithm>
#include <iostream>
#include <cstdio>
#include <cstdlib>
#include <cmath>
#include <queue>
#include <map>
#define INF 100000000
#define rep(i,a) for(int i=0;i<(a);i++)

using namespace std;
typedef long long ll;


int main(){
    int n;
    cin>>n;
    
    rep(i,n){
        int x[100]={},y[100]={};
        string a,b;
        cin>>a>>b;
        if(a.size()>80 || b.size()>80){cout<<"overflow"<<endl;return 0;}
        else{
        for(int i=a.size()-1;i>=0;i--){
            x[a.size()-1-i]=a[i]-'0';
        }
        for(int i=b.size()-1;i>=0;i--){
            y[b.size()-1-i]=b[i]-'0';
        }
        int ans[100]={};
        for(int i=0;i<80;i++){
            ans[i]+=x[i]+y[i];
            ans[i+1]+=ans[i]/10;
            ans[i]-=ans[i]/10*10;
        }
        if(ans[80]!=0)cout<<"overflow"<<endl;
        else{
            bool flag=false;
            for(int i=79;i>=0;i--){
                if(ans[i]!=0)flag=true;
                if(flag||i==0)cout<<ans[i];
            }
        }
        cout<<endl;
        }
    }
    return 0;
}

C++: #include <cmath>
#include <iostream>
#include <string>
#include <cstdlib>

using namespace std;
int main(){
    string inp[2];
    int output[81],n;
    cin >> n;
    for(int j=0;j<n;j++){
        fill(output,output+81,0);
        cin >> inp[0] >> inp[1];
        if(inp[0].length()>80||inp[1].length()>80) cout << "overflow" << endl;
        else{
            for(int i=0;i<81;i++){
                if((int)inp[0].length()-1-i >= 0){
                    output[i]+=inp[0][inp[0].length()-1-i]-'0';
                }
                if((int)inp[1].length()-1-i >= 0){
                    output[i]+=inp[1][inp[1].length()-1-i]-'0';
                }
                if(output[i]>9){
                    output[i]%=10;
                    output[i+1]+=1;
                }
            }
            if(output[80]>0) cout << "overflow" << endl;
            else{
                bool flag=false;
                for(int i=79;i>=0;i--){
                    if(!flag&&output[i]!=0) flag=true;
                    if(flag){
                        cout << output[i];
                    }
                }
                cout << endl;
            }
        }
    }
    return 0;
}

C++: #include <iostream>
#include <string>
#include <stack>
#include <cstdlib>

using namespace std;

int main() {
  int N;
  
  cin >> N;

  for (int N_i = 0; N_i < N; N_i++) {
    bool overflow = false;
    string str_data1, str_data2;
    cin >> str_data1;
    cin >> str_data2;

    stack<int> data1, data2; 
    for (int data1_i = 0; data1_i < str_data1.size(); data1_i++) {
      char buff[2];
      buff[0] = str_data1[data1_i];
      buff[1] = '\0';
      data1.push(atoi(buff));
    }
    if (data1.size() > 80) {
      cout << "overflow" << endl;
      continue;
    }
    for (int data2_i = 0; data2_i < str_data2.size(); data2_i++) {
      char buff[2];
      buff[0] = str_data2[data2_i];
      buff[1] = '\0';
      data2.push(atoi(buff));
    }
    if (data2.size() > 80) {
      cout << "overflow" << endl;
      continue;
    }

    stack<int> sum;
    int carry = 0;
    int flag = data1.size() < data2.size() ? 0 : 1;
    while(flag == 0 ? !data1.empty() : !data2.empty()) {
      int presum = data1.top() + data2.top() + carry;
      carry = presum / 10;
      sum.push(presum % 10);
      data1.pop();
      data2.pop();
    }
    stack<int> *data;
    data = data1.size() > data2.size() ? &data1: &data2;
    while(!data->empty()) {
      int presum = data->top() + carry;
      carry = presum / 10;
      sum.push(presum % 10);
      data->pop();
    }

    if (carry != 0)
      sum.push(carry);

    if (sum.size() > 80) {
      cout << "overflow" << endl;
      continue;
    }

    if (overflow) {
      cout << overflow << endl;
    } else {
      for (int sum_i = 0; !sum.empty(); sum_i++) {
        cout << sum.top();
        sum.pop();
      }
      cout << endl;
    }
  }

  return 0;
}

C++: #include<iostream>
#include<string>
using namespace std;
int main(){
  int n;
  cin >>n;
  while(n--){
    string a,b,ans;
    cin >>a>>b;
    int as = a.size(), bs = b.size(), ms = (as>bs)? as:bs,t=0;
    for(int i=0; i<ms; i++){
      if(i<as && i<bs){t = a[as-i-1]+b[bs-i-1]+t/10-2*'0';}
      else if(i<as){t = a[as-i-1]+t/10-'0';}
      else if(i<bs){t = b[bs-i-1]+t/10-'0';}
      ans+=(char)(t%10)+'0';
    }
    if(t/10){ans+='1';}
    if(ans.size()>80){cout <<"overflow"<<endl;}
    else{for(int i=0; i<ans.size(); i++){cout <<ans[ans.size()-i-1];}cout <<endl;}
  }
  return 0;
}