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 <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)
      overflow = true;
    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)
      overflow = true;

    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)
      overflow = true;

    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>
#include <algorithm>

using namespace std;

char c[10] = {'0', '1', '2', '3', '4', '5', '6', '7', '8', '9'};

int main() {
	int n; cin >> n;
	string s1, s2;
	
	for (int i=0; i<n; i++) {
		cin >> s1 >> s2;
		reverse(s1.begin(), s1.end());
		reverse(s2.begin(), s2.end());
		
		string s = "";
		
		if (s1.size() == s2.size()) {
			int r = 0, t = 0;
			for (int i=0; i<(int)s1.size(); i++) {
				int x = s1[i]-c[0], y = s2[i]-c[0];
				int z = x + y + t;
				r = z%10;
				t = z/10;
				s += c[r];
			}
			if (t) s += c[t];
		} else if (s1.size() > s2.size()) {	
			int r = 0, t = 0;
			for (int i=0; i<(int)s2.size(); i++) {
				int x = s1[i]-c[0], y = s2[i]-c[0];
				int z = x + y + t;
				r = z%10;
				t = z/10;
				s += c[r];
			}
			if (t) {
				for (int i=(int)s2.size(); i<(int)s1.size(); i++) {
					int z = s1[i]-c[0]+t;
					r = z%10;
					t = z/10;
					s += c[r];
				}
				if (t) s += c[t];
			} else {
				for (int i=(int)s2.size(); i<(int)s1.size(); i++) s += c[s1[i]-c[0]];
			}
		} else {
			int r = 0, t = 0;
			for (int i=0; i<(int)s1.size(); i++) {
				int x = s1[i]-c[0], y = s2[i]-c[0];
				int z = x + y + t;
				r = z%10;
				t = z/10;
				s += c[r];
			}
			if (t) {
				for (int i=(int)s1.size(); i<(int)s2.size(); i++) {
					int z = s2[i]-c[0]+t;
					r = z%10;
					t = z/10;
					s += c[r];
				}
				if (t) s += c[t];
			} else {
				for (int i=(int)s1.size(); i<(int)s2.size(); i++) s += c[s2[i]-c[0]];
			}
		}
		
		if (s.size() > 80) cout << "overflow" << endl;
		else {
			reverse(s.begin(), s.end());
			cout << s << endl;
		}
	}
	
	return 0;
}

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

int main(){
    ios::sync_with_stdio(false);
    int n;
    for(cin>>n;n--;){
        int k=0;
        string a,b;
        cin >> a;
        cin >> b;
        if(a.size() < b.size()) swap(a,b);//????????????a>b?????????
        while(b.size() < a.size()) b = '0' + b;//????????????a==b?????????
        for(int i=a.size()-1;i>=0;i--){//???????????????????????????
            a[i] += b[i] - '0' + k;
            if(a[i] > '9'){
                a[i]-=10;
                k = 1;
            }
            else k = 0;
        }
        if(k)   a="1"+a;//??°??????????????????????????????????¶?????????¨???
        if(a.size() > 80)   cout << "overflow" << "\n";
        else    cout << a << "\n";
    }
    return 0;
}

C++: #include<iostream>
#include<cstring>
using namespace std;
int main(){
	int n;
	cin >>n;
	for(int i=0; i<n; i++){
		char num[2][90];
		int ans[90],up=0,t,l;
		cin >>num[0]>>num[1];
		int a = strlen(num[0]), b = strlen(num[1]), max = (b>a)? b:a, min = (b>a)? a:b;
		for(l=0; l<max; l++){
			if(l<min){t = (int)num[0][a-l-1]+(int)num[1][b-l-1]-48*2+up;}
			else if(a<b){t = (int)num[1][b-l-1]-48+up;}
			else{t = (int)num[0][a-l-1]-48+up;}
			if(t>9){t-=10;up = 1;}
			else{up = 0;}
			ans[l] = t;
		}
		if(up == 1){ans[max] = 1;l++;}
		if(l<=80)for(int m=l-1; m>=0; m--){cout <<ans[m];}
		else{cout<<"overflow";}
		cout <<endl;
	}
	return 0;
}