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 <iostream>
#include <string>
#include <sstream>
#include <deque>
class BigInt
{
  std::deque<char> m_digits;
public:
  BigInt(std::string str) {
    for(auto it = str.begin(); it != str.end(); ++it) {
      m_digits.push_back(*it - '0');
    }
  }

  BigInt(int num) {
    std::ostringstream oss;
    oss << num;
    BigInt(oss.str());
  }

  std::string str() const {
    if (m_digits.size() > 80) {
      return "overflow";
    }
    std::string str;
    for (auto it = m_digits.begin(); it != m_digits.end(); ++it) {
      str += *it + '0';
    }
    return str;
  }

  const BigInt& operator +=(const BigInt& rhs) {
    std::deque<char> new_digits;
    auto lhs_rit = m_digits.rbegin();
    auto rhs_rit = rhs.m_digits.rbegin();

    char inc = 0;
    while (lhs_rit != m_digits.rend() &&
           rhs_rit != rhs.m_digits.rend()) {
      char temp = *lhs_rit + *rhs_rit;
      temp += inc;
      if (temp >= 10) {
        inc = temp / 10;
        temp -= inc * 10;
      }
      new_digits.push_front(temp);
      ++lhs_rit;
      ++rhs_rit;
    }

    if (lhs_rit != m_digits.rend()) {
      while (lhs_rit != m_digits.rend()) {
        char temp = *lhs_rit + inc;
        if (temp >= 10) {
          inc = temp / 10;
          temp -= inc * 10;
        } else {
          inc = 0;
        }
        new_digits.push_front(temp);
        ++lhs_rit;
      }
    }
    else if (rhs_rit != rhs.m_digits.rend()) {
      while (rhs_rit != rhs.m_digits.rend()) {
        char temp = *rhs_rit + inc;
        if (temp >= 10) {
          inc = temp / 10;
          temp -= inc * 10;
        } else {
          inc = 0;
        }
        new_digits.push_front(temp);
        ++rhs_rit;
      }
    }
    if (inc > 0) new_digits.push_front(inc);
    m_digits = new_digits;
  }
};

BigInt operator +(const BigInt& lhs, const BigInt& rhs)
{
  BigInt result(lhs);
  result += rhs;
  return result;
}

int main()
{
  int n;
  std::cin >> n;
  while (n--) {
    std::string num;
    std::cin >> num;
    BigInt num1(num);
    std::cin >> num;
    BigInt num2(num);
    
    BigInt num3 = num1 + num2;
    std::cout << num3.str() << std::endl;
  }
  return 0;
}

C++: #include<bits/stdc++.h>
using namespace std;
 
signed main(){
int a;
cin>>a;
for(int b=0;b<a;b++){
    int goukei[81]{};
    char c[110]{},d[110]{};
    scanf("%s%s",c,d);
    int e,f;
    e=strlen(c);f=strlen(d);
    if(e>80||f>80)puts("overflow");
   else{ e--;f--;
    for(int g=80;g!=0;g--,e--,f--){
	int s=goukei[g];
	if(e>-1)s+=c[e]-48;
	if(f>-1)s+=d[f]-48;
	goukei[g-1]=s/10;
	goukei[g]=s%10;
	}
	if(goukei[0])puts("overflow");
	else{
	    bool ss=false;
	    for(int m:goukei){
		if(m)ss=true;
		if(ss)printf("%d",m);
		}
		puts("");
	    }}
    }
}

C++: #define _CRT_SECURE_NO_WARNINGS
#define M_PI 3.14159265358979
#include <iostream>
#include <stdio.h>
#include <string>
#include <math.h>
#include<vector>
#include<cmath>
#include <algorithm>

using namespace std;

int main(){
	int n;
	string x, y;
	int a, b, c;
	cin >> n;
	for (int i = 0; i < n; i++){
		cin >> x >> y;
		if (x.length() > 80 || y.length() > 80){
			cout << "overflow" << endl;
		}
		else{
			a = stoi(x);
			b = stoi(x);
			if (to_string(a + b).length() > 80){
				cout << "overflow" << endl;
			}
			else cout << a + b << endl;
		}
	}
	return 0;
}

C++: #include<bits/stdc++.h>
#include<vector>
#include<list>
#include<stack>
#include<queue>
#include<algorithm>
using namespace std;


int main(){
    int t;
    scanf("%d",&t);
    while(t--){
        char c1[1000],c2[1000];
        scanf("%s",&c1);
        scanf("%s",&c2);
        string s1,s2;
        s1=c1, s2=c2;
        if(s1=="0" && s2=="0"){
            printf("0\n");
            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){
            printf("overflow\n");
        }else{
            printf("%s\n",res2.c_str());
        }
    }
    return 0;
}