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>
using namespace std;

int charToInt(char);
string sumStr(string, string);

int charToInt(char c) {
  return (int)c - (int)'0';
}

string sumStr(string a, string b) {
  int pa = a.length()-1, pb = b.length()-1;
  string str(83, '0');
  int pstr = str.length()-1;
  int next = 0;
  while(pa >= 0 && pb >= 0) {
    int temp = charToInt(a[pa]) + charToInt(b[pb]) + next;
    next = temp / 10;
    str[pstr] = (char)(temp%10+(int)'0');

    pa--;
    pb--;
    pstr--;
  }
  while(pa >= 0) {
    int temp = charToInt(a[pa]) + next;
    next = temp / 10;
    str[pstr] = (char)(temp%10+(int)'0');

    pa--;
    pstr--;
  }
  while(pb >= 0) {
    int temp = charToInt(b[pb]) + next;
    next = temp / 10;
    str[pstr] = (char)(temp%10+(int)'0');

    pb--;
    pstr--;
  }
  str[pstr] = (char)(next + (int)'0');

  for(int i = 0; i < str.length(); i++) {
    if(str[i] != '0') {
      return str.substr(i, str.length()-i);
    }
  }
  return "0";
}

int main() {
  int N;
  cin >> N;
  for(int i = 0; i < N; i++) {
    bool overflow = false;
    int data[N][8];
    string sum;
    for(int j = 0; j < 2; j++) {
      string str;
      cin >> str;

      if(!overflow) {
	if(j == 0) sum = str;
	else       sum = sumStr(sum, str);
        if(sum.length() > 80) overflow = true;
      }
    }

    if(overflow) cout << "overflow" << endl;
    else         cout << sum << endl;
  }
  return 0;
}

C++: #define _CRT_SECURE_NO_WARNINGS
#define M_PI 3.14159265358979
#include <iostream>
#include <stdio.h>
#include <string>
#include <math.h>
#include<iomanip>
#include<vector>
#include<cmath>
#include <algorithm>
#include <cstdlib>
#include <typeinfo>
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(y);
			if (to_string(a + b).length() > 80)
			{
				cout << "overflow" << endl;
			}
			else cout << a + b << endl;
		}
	}
}

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

int main() {
	int n,x1,x2,y1,y2,p,q,a=0,b=0;
	vector<int> E,F;
	vector<long long int> S, T;
	vector<vector<long long int>> I,J;
	string s, t;
	vector<string> U;
	vector<vector<string>> UU;

	cin >> n;
	for (int k = 0; k < n; k++) {
		cin >> s >> t;
		int f = 0;
		if (s.size() > 80 || t.size() > 80) {
			f = 1;
			F.push_back(f);
		}
		else F.push_back(0);
		x1 = s.size() % 10; y1 = (s.size()+9) / 10;p = 8 - y1;
		for (int i = 0; i < p; i++)S.push_back(0);
		if (x1 == 0) for (int i = 0; i < y1; i++)S.push_back(stoll(s.substr(i * 10, 10)));
		else {
			S.push_back(stoll(s.substr(0, x1)));
			for (int i = 1; i < y1; i++)S.push_back(stoll(s.substr(x1 + (i-1) * 10, 10)));
		}

		x2 = t.size() % 10; y2 = (t.size()+9) / 10;q = 8 - y2;
		for (int i = 0; i < q; i++)T.push_back(0);
		if (x2 == 0) for (int i = 0; i < y2; i++)T.push_back(stoll(t.substr(i * 10, 10)));
		else {
			T.push_back(stoll(t.substr(0, x2)));
			for (int i = 1; i < y2; i++)T.push_back(stoll(t.substr(x2 + (i-1) * 10, 10)));
		}
		I.push_back(S); S.clear();
		J.push_back(T); T.clear();
	}
	for (int k = 0; k < I.size(); k++) {
		for (int i = 0; i < 8; i++) U.push_back(to_string(I[k][i] + J[k][i]));
		UU.push_back(U); U.clear();
	}
	
	for (int k = 0; k < I.size(); k++) {
		for (int i = 7; i >= 0; i--) {
			UU[k][i] = to_string(stoll(UU[k][i]) + b);
			if (UU[k][i].size() == 11 && i == 0) {
				UU[k][i] = "a";
				b = 1;
			}
			else if (UU[k][i].size() == 11) {
				UU[k][i] = UU[k][i].substr(1, 10);
				b = 1;
			}
			else if (UU[k][i].size() < 10) {
				for (int j = 0; j < 10 - UU[k][i].size()+j; j++) UU[k][i] = "0" + UU[k][i];
				b = 0;
			}
			else b = 0;
		}
		b = 0;
	}

	for (int k = 0; k < UU.size(); k++) {
		int e = 0;
		for (int i = 0; i < UU[k].size(); i++) {
			if (UU[k][i] == "a") {
				e = 100;
				break;
			}
			else if (stoll(UU[k][i]) == 0);
			else {
				e = i;
				E.push_back(e);
				break;
			}
		}
	}

		for(int k=0;k<UU.size();k++){
			if (E[k] == 100)cout << "overflow" << endl;
			else if (F[k] == 1)cout << "overflow" << endl;
			else {
				cout << stoll(UU[k][E[k]]);
				for (int i = E[k]+1; i < UU[k].size(); i++) cout << UU[k][i];
			}
			cout << endl;
		}
	return 0;
}