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>
#define MAX 79

#include <string>

using namespace std;

int main()
{
	int n;
	cin >> n;
	for (int counter = 0; counter < n; counter++){
		string a, b;
		cin >> a >> b;
		if (a.length() > MAX || b.length() > MAX){
			cout << "overflow" << endl;
			continue;
		}
		int inta[MAX + 2], intb[MAX + 2], sum[MAX + 2];
		fill(inta, inta + MAX + 2, 0);
		fill(intb, intb + MAX + 2, 0);
		fill(sum, sum + MAX + 2, 0);
		for (int i = 0; i < (signed)a.length(); i++){
			inta[i] = a[a.length() - i - 1] - '0';
		}
		for (int i = 0; i < (signed)b.length(); i++){
			intb[i] = b[b.length() - i - 1] - '0';
		}

		int over = 0;
		for (int i = 0; i < MAX + 2; i++){
			sum[i] = inta[i] + intb[i] + over;
			if (sum[i] >= 10){
				sum[i] -= 10;
				over = 1;
			}
			else{
				over = 0;
			}
		}

		for (int i = MAX + 1;; i--){
			if (sum[i] != 0){
				if (i >= MAX){
					cout << "overflow" << endl;
					break;
				}
				for (int j = i; j >= 0; j--){
					cout << sum[j];
				}
				cout << endl;
				break;
			}
		}
	}
	return 0;
}

C++: #include<cstdio>
#include<iostream>
#include<vector>
#include<algorithm>
using namespace std;
int main() {
	int n;
	string a,b;
	cin >> n;
	while(n--) {
		cin >> a >> b;
		string ret = "";
		reverse(a.begin(),a.end());
		reverse(b.begin(),b.end());
		int a_len = a.size(),b_len = b.size();
		int num = max(a_len,b_len),carry = 0,v;
		char tmp[1];
		for(int i = 0; i < num; i++) {
			if(a_len <= i || b_len <= i) {
				if(b_len <= i) v = carry + a[i] - '0';
				else v = carry + b[i] - '0';
			}
			else v = carry + a[i] - '0' + b[i] - '0';
			carry = v / 10;
			sprintf(tmp,"%d",v % 10);
			ret += tmp;
		}
		reverse(ret.begin(),ret.end());
		cout << (ret.size() > 80 ? "overflow" : ret) << endl;
	}
	return 0;
}

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

using namespace std;

int main() {
    int N;
    cin >> N;
    for (int i=0; i<N; i++) {
        string a, b;
        cin >> a >> b;
        int sum, ret=0;
        if (a.length() < b.length()) swap(a,b);
        int l = b.length()-1;
        for (int i=a.length()-1; i>=0; i--) {
            if (l >= 0) {
                sum = (a[i]-'0')+(b[l]-'0')+ret;
                l--;
            } else {
                sum = (a[i]-'0')+ret;
            }
            if (sum>=10) {
                ret = 1;
                sum -= 10;
            } else {
                ret = 0;
            }
            a[i] = sum+'0';
        }
        if (ret != 0) {
            a.insert(a.begin(), ret+'0');
        }
        if (a.length() > 80) cout << "overflow" << endl;
        else cout << a << endl;
    }
    return 0;
}

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

int reverse(string &s){
	int n = s.size(), n2 = n/2;
	for (int i=0; i<n2; i++)
		swap(s[i], s[n-1-i]);
	return 0;
}

int main(void){
	int n;
	cin>>n;
	for (int i=0; i<n; i++){
		string s1, s2, s3="";
		cin>>s1>>s2;

		reverse(s1);
		reverse(s2);

		while (s1.size()<s2.size()) s1 += '0';
		while (s2.size()<s1.size()) s2 += '0';

		int c=0, stmp;
		for (int i=0; i<s1.size(); i++){
			stmp = s1[i]-'0' + s2[i]-'0' + c;
			c = stmp / 10;
			s3 += stmp%10 +'0';
		}
		if (c!=0) s3 += c + '0';

		reverse(s3);
		if (s3.size()>80)
			cout<<"overflow"<<endl;
		else
			cout<<s3<<endl;

	}
	return 0;
}

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

using namespace std;

int main(){
    int n; cin >> n;
    while(n--){
        string a,b;
        cin >> a >> b;
        reverse(a.begin(), a.end());
        reverse(b.begin(), b.end());
        if(b.size() < a.size()) swap(a,b);
        b += "0";
        for(int i=0; i<b.size()-1; i++){
            int aa = i < a.size() ? a[i] - '0' : 0;
            int bb = b[i] - '0';
            int cc = aa + bb;
            b[i] = (cc%10) + '0';
            //cout << aa << "+" << bb << "=" << cc << endl;
            if(cc >= 10){
                b[i+1]++;
            }
        }
        if(b[b.size()-1] == '0')
            b = string(b.begin(), b.end()-1);
        reverse(b.begin(), b.end());
        if(b.size() > 80)
            cout << "overflow" << endl;
        else cout << b << endl;
    }
    return 0;
}

C++: #include<iostream>
#include<stdio.h>
#include <algorithm>
#include<string.h>
#include <utility>
using namespace std;
int main(){
	int m;
	string in,in2;
	int a[1000],b[1000],ans[1000];
	int f;
	int alen,blen,mlen;
	cin>>m;
	while(m--){
		for(int i=0;i<90;i++){
			a[i]=b[i]=ans[i]=0;
		}
		cin>>in;
		cin>>in2;
		alen=in.size();
		blen=in2.size();
		for(int i=0;i<alen;i++){
			a[alen-1-i]=in[i]-'0';
		}
		for(int i=0;i<blen;i++){
			b[blen-1-i]=in[i]-'0';
		}
		mlen=max(alen,blen);
		if(mlen>80){
			cout<<"overflow"<<endl;
			continue;
		}
		f=0;
		for(int i=0;i<mlen+1;i++){
			ans[i]=(a[i]+b[i]+f)%10;
			f=(a[i]+b[i]+f)/10;
		}
		if(ans[mlen]!=0){
			if(mlen>79){
				cout<<"overflow"<<endl;
				continue;
			}
			cout<<ans[mlen];
		}
		
		for(int i=0;i<mlen;i++){
			cout<<ans[mlen-1-i];
		}cout<<endl;
		
		
	}
	return 0;
}