Go: package main

import (
	"bufio"
	"fmt"
	"math"
	"os"
	"sort"
	"strconv"
	"strings"
)

func main() {
	io := NewIo()
	defer io.Flush()
	for {
		n, p := io.NextInt(), io.NextInt()
		if n == -1 {
			break
		}
		isPrime := SieveOpt(200000)
		prime := make([]int, p)
		cnt := 0
		if n < 2 {
			prime[0] = 2
			cnt++
		}
		for i := (n + 1) / 2; i < len(isPrime) && cnt < p; i++ {
			if isPrime[i] == true {
				prime[cnt] = 2*i + 1
				cnt++
			}
		}
		comb := make([]int, 0, p*p)
		for i := 0; i < p; i++ {
			for j := i; j < p; j++ {
				comb = append(comb, prime[i]+prime[j])
			}
		}
		sort.Ints(comb)
		io.Println(comb[p-1])
	}
}

func SieveOpt(x int) []bool {
	flags := make([]bool, x/2) // flags[i] := 2*j+1 is prime
	for i := 1; i < x/2; i++ {
		flags[i] = true
	}
	sqrtXi := int(math.Ceil(math.Sqrt(float64(x)))) / 2
	for i := 1; i < sqrtXi; i ++ {
		if !flags[i] {
			continue
		}
		for j := 2 * i * (i + 1); j < x/2; j += 2*i + 1 {
			flags[j] = false
		}
	}
	return flags
}

// I/O template
type Io struct {
	reader    *bufio.Reader
	writer    *bufio.Writer
	tokens    []string
	nextToken int
}

func NewIo() *Io {
	return &Io{
		reader: bufio.NewReader(os.Stdin),
		writer: bufio.NewWriter(os.Stdout),
	}
}

func (io *Io) Flush() {
	err := io.writer.Flush()
	if err != nil {
		panic(err)
	}
}

func (io *Io) NextLine() string {
	var buff []byte
	for {
		line, isPrefix, err := io.reader.ReadLine()
		if err != nil {
			panic(err)
		}
		buff = append(buff, line...)
		if !isPrefix {
			break
		}
	}
	return string(buff)
}

func (io *Io) Next() string {
	for io.nextToken >= len(io.tokens) {
		line := io.NextLine()
		io.tokens = strings.Fields(line)
		io.nextToken = 0
	}
	s := io.tokens[io.nextToken]
	io.nextToken++
	return s
}

func (io *Io) NextInt() int {
	i, err := strconv.Atoi(io.Next())
	if err != nil {
		panic(err)
	}
	return i
}

func (io *Io) NextFloat() float64 {
	i, err := strconv.ParseFloat(io.Next(), 64)
	if err != nil {
		panic(err)
	}
	return i
}

func (io *Io) Println(a ...interface{}) {
	fmt.Fprintln(io.writer, a...)
}

func (io *Io) Printf(format string, a ...interface{}) {
	fmt.Fprintf(io.writer, format, a...)
}

func (io *Io) PrintIntSlice(a []int) {
	b := []interface{}{}
	for _, x := range a {
		b = append(b, x)
	}
	io.Println(b...)
}

func (io *Io) PrintStringSlice(a []string) {
	b := []interface{}{}
	for _, x := range a {
		b = append(b, x)
	}
	io.Println(b...)
}

func Log(name string, value interface{}) {
	fmt.Fprintf(os.Stderr, "%s=%+v\n", name, value)
}

func ExampleIo() {
	io := NewIo()
	defer io.Flush()
	a := make([]int, 0, 3)
	a[0], a[1], a[2] = io.NextInt(), io.NextInt(), io.NextInt()
	io.Println(a)
	io.PrintIntSlice(a)
	Log("io", a)
}

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

const int MAX_N = 210000;

// p[i] := i 番目の素数
vector<int> p;
// is_p[i] := i が素数のとき true
bool is_p[MAX_N+1] = {false,false};

// エラトステネスのふるい
void sieve(){
	for(int i=2 ; i <= MAX_N ; i++ )
		is_p[i] = true;
	for(int i=2 ; i*i <= MAX_N ; i++ ){
		if( is_p[i] ){
			p.push_back( i );
			for(int j = i+i ; j <= MAX_N ; j += i ){
				is_p[j] = false;
			}
		}
	}
}

// N より大きい任意の2つ素数を足したものの中で P 番目に小さい数値を返す
int M(int N, int P){
	int id;
	for(int i=0 ; i < p.size() ; i++ ){
		if( p[i] > N ){
			id = i;
			break;
		}
	}
	priority_queue<int> q;
	//vector<int> v;
	for(int i = id ; i < id+P ; i++ ){
		for(int j = i ; j < id+P ; j++ ){
			q.push( -p[i] - p[j] );
			//v.push_back( p[i] + p[j] );
		}
	}
	//sort( v.begin() , v.end() );
	//if( P-1 >= v.size() ) return v[v.size()-1];
	//return v[P-1];
	int result;
	for(int i=0 ; i < P ; i++ ){
		result = q.top(); q.pop();
	}
	return -result;
}

int main(){
	int n,p;
	sieve();
	while( cin >> n >> p , n || p ){
		cout << M(n,p) << endl;
	}
}

C++: #include <cstdio>
#include <cmath>
#include <algorithm>
#include <vector>
 
using namespace std;
 
const int MAX_N = 100000;
const int MAX_P = 100;
const int MAX_PRIME = (MAX_N+MAX_P)*2+100;
 
int prime[MAX_N];
int n_prime;
 
void init_prime() {
  prime[0] = 2;
  n_prime = 1;
   for (int i=3;i<MAX_PRIME;i++) {
    if (i%2 == 0) continue;
    bool ok = true;
    for (int j=3;j<=sqrt(i);j+=2) {
      if (i%j == 0) {
        ok = false;
        break;
      }
    }
    if (ok) prime[n_prime++] = i;
  }
}
 
int main() {
  init_prime();
  while (true) {
    int N, P;
    scanf("%d %d", &N, &P);
    if (N == -1 && P == -1) break;
    const int* p = upper_bound(prime, prime+n_prime, N);
    vector<int> t;
    for (int i=0;i<P;i++) {
      for (int j=i;j<P;j++) t.push_back(p[i]+p[j]);
    }
    sort(t.begin(), t.end());
    printf("%d\n", t[P-1]);
  }
  return 0;
}

C++: #include "bits/stdc++.h"
#define pb push_back
#define mt make_tuple
#define all(a) (a).begin(),(a).end()
#define frep(i,a,n) for(int i=(a);i<(n);++i)
#define rep(i, n) for(int i=0;i<(n);++i)
#define rrep(i, n) for(int i=n-1;i>=0;--i)
#define debug(x) cout << #x << " : " << x << " ------------ " << endl;
using namespace std;
int main()
{
    int MAX = 200000;
    bool tmp[MAX];
    fill(tmp, tmp + MAX, true);
    tmp[0] = false;
    tmp[1] = false;
    for (int i = 4; i < MAX; i += 2) {
        tmp[i] = false;
    }
    for (int i = 3; i < MAX; i += 2) {
        if (!tmp[i]) continue;
        for (int j = 2; i * j < MAX; ++j) {
            tmp[i*j] = false;
        }
    }
    vector<int> prime{2};
    for (int i = 3; i < MAX; i += 2) {
        if (tmp[i]) prime.pb(i);
    }
    
    int n, p;
    while (cin >> n >> p, n != -1 && p != -1) {
        vector<int> sums;
        auto it = upper_bound(all(prime), n);
        rep (i, p) {
            frep (j, i, p) {
                sums.pb(*(it+i) + *(it+j));
            }
        }
        sort(all(sums));
        cout << sums[p-1] << endl;
    }
    return EXIT_SUCCESS;
}