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 <bits/stdc++.h>

#define rep(i, n) for (int i = 0; i < int(n); i++)
#define rrep(i, n) for (int i = int(n) - 1; i >= 0; i--)
#define reps(i, n) for (int i = 1; i <= int(n); i++)
#define rreps(i, n) for (int i = int(n); i >= 1; i--)
#define repc(i, n) for (int i = 0; i <= int(n); i++)
#define rrepc(i, n) for (int i = int(n); i >= 0; i--)
#define repi(i, a, b) for (int i = int(a); i < int(b); i++)
#define repic(i, a, b) for (int i = int(a); i <= int(b); i++)
#define each(x, y) for (auto &x : y)
#define all(a) (a).begin(), (a).end()
#define bit(b) (1ll << (b))

using namespace std;

using i32 = int;
using i64 = long long;
using u64 = unsigned long long;
using f80 = long double;
using vi32 = vector<i32>;
using vi64 = vector<i64>;
using vu64 = vector<u64>;
using vf80 = vector<f80>;
using vstr = vector<string>;

inline void yes() { cout << "Yes" << '\n'; exit(0); }
inline void no() { cout << "No" << '\n'; exit(0); }
inline i64 gcd(i64 a, i64 b) { if (min(a, b) == 0) return max(a, b); if (a % b == 0) return b; return gcd(b, a % b); }
inline i64 lcm(i64 a, i64 b) { return a / gcd(a, b) * b; }
inline u64 xorshift() { static u64 x = 88172645463325252ull; x = x ^ (x << 7); return x = x ^ (x >> 9); }
void solve(); int main() { ios::sync_with_stdio(0); cin.tie(0); cout << fixed << setprecision(16); solve(); return 0; }
template <typename T> class pqasc : public priority_queue<T, vector<T>, greater<T>> {};
template <typename T> class pqdesc : public priority_queue<T, vector<T>, less<T>> {};
template <typename T> inline void amax(T &x, T y) { if (x < y) x = y; }
template <typename T> inline void amin(T &x, T y) { if (x > y) x = y; }
template <typename T> inline T power(T x, i64 n, T e = 1) { T r = e; while (n > 0) { if (n & 1) r *= x; x *= x; n >>= 1; } return r; }
template <typename T> istream& operator>>(istream &is, vector<T> &v) { each(x, v) is >> x; return is; }
template <typename T> ostream& operator<<(ostream &os, vector<T> &v) { rep(i, v.size()) { if (i) os << ' '; os << v[i]; } return os; }
template <typename T, typename S> istream& operator>>(istream &is, pair<T, S> &p) { is >> p.first >> p.second; return is; }
template <typename T, typename S> ostream& operator<<(ostream &os, pair<T, S> &p) { os << p.first << ' ' << p.second; return os; }

void solve() {
  int N; cin >> N;
  rep(i, N) {
    string s, t;
    cin >> s >> t;
    if (s.size() > 80 || t.size() > 80) {
      cout << "overflow" << '\n';
      continue;
    }
    if (s == "0") {
      cout << t << '\n';
      continue;
    } else if (t == "0") {
      cout << s << '\n';
      continue;
    }
    string ans = "";
    bool up = false;
    rep(i, min(s.size(), t.size())) {
      int a = s[s.size() - 1 - i] - '0';
      int b = t[t.size() - 1 - i] - '0';
      int x = up ? 1 : 0;
      up = false;
      x += a + b;
      if (x >= 10) up = true, x -= 10;
      string te;
      te += (char) x + '0';
      ans = te + ans;
    }
    repi(i, min(s.size(), t.size()), max(s.size(), t.size())) {
      int a;
      if (i >= s.size()) a = t[t.size() - i - 1] - '0';
      else a = s[s.size() - i - 1] - '0';
      int x = up ? 1 : 0;
      up = false;
      x += a;
      if (x >= 10) up = true, x -= 10;
      string te;
      te += (char) x + '0';
      ans = te + ans;
    }
    if (up) {
      ans = "1" + ans;
    }
    if (ans.size() > 80) {
      cout << "overflow" << '\n';
      continue;
    }
    cout << ans << '\n';
  }
}

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

int main(){
  std::string as;
  std::string bs;
  int a[102]={0};
  int b[102]={0};
  int sum[102]={0};
  int a_size,b_size,sum_size,n,i,j,flag=0,overflow=0;
  char num;

  std::cin >> n;

  for(i=0;i<n;i++){
    std::cin >> as >> bs;
    a_size = as.size();
    b_size = bs.size();

    for(j=0;j<102;j++){   //??????????????????(100??????00100)
      if(101-j<=a_size){
        a[101-j] = as[j-a_size] - '0';
      }else{
        a[101-j] = 0;
      }
      if(101-j<=b_size){
        b[101-j] = bs[j-a_size] - '0';
      }else{
        b[101-j] = 0;
      }
    }

    for(j=0;j<102;j++){       //sum????¨????
      sum[j] = a[j] + b[j];
      if(flag){               //??°?????????
        sum[j] += 1;
        flag = 0;
      }
      if(j>=80&&sum[j]!=0){   //???????????????????????????
        overflow = 1;
        break;
      }
      if(sum[j] > 9){         //??°???????????????
        sum[j] -= 10;
        flag = 1;
      }
    }
    if(overflow){             //??????
      std::cout << "overflow" << std::endl;
      overflow = 0;
      continue;
    }
    for(j=0;j<102;j++){
      if(sum[101-j]!=0){
        sum_size = 101-j;
        break;
      }
    }
    for(j=sum_size;j>=0;j--){
      num = sum[j] + '0';
      std::cout << num;
    }
    std::cout << std::endl;
  }
}