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++: //#define _GRIBCXX_DEBUG
#include <bits/stdc++.h>
using namespace std;

// ?????¬???????????¬??????
#pragma region MACRO
#define P(x) cout << (x) << endl
#define p(x) cout << (x)
#define PED cout << "\n"
#define rep(i,n) for(int i=0; i<(int)n; ++i)
#define REP(i,x,n) for(int i=x; i<(int)n; ++i)
#define repi(i,n) for(int i=0; i<=(int)n; ++i)
#define REPI(i,x,n) for(int i=x; i<=(int)n; ++i)
#define ILP while(true)
#define FOR(i,c) for(__typeof((c).begin())!=(c).begin(); i!=(c).end(); ++i)
#define ALL(c) (c).begin(), (c).end()
#define mp make_pair
#pragma endregion

// ???
#pragma region TYPE_DEF
typedef long long ll;
typedef pair<int, int> pii;
typedef pair<string, string> pss;
typedef pair<string, int> psi;
typedef pair<int, string> pis;
typedef vector<int> vi;
typedef vector<double> vd;
typedef vector<long double> vld;
typedef vector<long> vl;
typedef vector<long long> vll;
typedef vector<string> vs;
#pragma endregion

// Effective std
#pragma region ESTD
template<typename C, typename T> int count(C& c, T t) { return count(ALL(c), t); }
template<typename C, typename F> int count_if(C& c, F f) { return count_if(ALL(c), f); }
template<typename C, typename T> void erase(C& c, T t) { remove(ALL(c), t), c.end(); }
template<typename C> void remove(vector<C>& c, unsigned int index) { c.erase(c.begin()+index); }
template<typename C, typename T, typename U> void replace(C& c, T t, U u) { replace(ALL(c), t, u); }
template<typename C, typename F, typename U> void replace_if(C& c, F f, U u) { (ALL(c), f, u); }
template<typename C> void reverse(C& c) { reverse(ALL(c)); }
template<typename C> void sort(C& c) { sort(ALL(c)); }
template<typename C, typename Pred> void sort(C& c, Pred p) { sort(ALL(c), p); }
#pragma endregion

// ?????°
#pragma region CONST_VAL
constexpr int PI = (2*acos(0.0));
constexpr int EPS = (1e-9);
constexpr int MOD = (int)(1e9+7);
constexpr int INF = 100000000;
#pragma endregion

void sieve(vector<int>& v, int n) {
    v.assign(n, 0);
    iota(ALL(v), 2);
    for(auto d=v.begin(); (*d)*(*d)<=v.back(); ++d)
        v.erase(remove_if(d+1, v.end(), [&](int x){return x%*d==0; }), v.end());
}

vector<int> primes;

int solve(int n, int p) {
    int k = upper_bound(ALL(primes), n) - primes.begin();
    vector<int> m;
    for(int i=k; i<k+p; ++i)
        for(int j=k; j<=i; ++j) m.push_back(primes[i] + primes[j]);
    sort(ALL(m));
    return m[p-1];
}

int main()
{
    sieve(primes, 120000);
    int n, p;
    while(cin>>n>>p, n!=-1) cout << solve(n, p) << endl;
    return 0;
}

C++: #include <cstdio>
#include <cstdlib>
#include <cmath>
#include <cstring>

#include <iostream>
#include <string>
#include <algorithm>
#include <vector>
#include <queue>
#include <stack>
#include <map>
#include <set>

#include <functional>
#include <cassert>

typedef long long ll;
using namespace std;

#define mod 1000000007 //1e9+7
#define INF 1000000000 //1e9
#define LLINF 2000000000000000000LL //2e18
#define SIZE 1001

const int MAX_P = 200000; //max
bool prime[MAX_P+1];

void Eratosthenes(void){
  for(int i=2;i<=MAX_P;i++)
    prime[i]=true;
  
  for(int i=2;i*i<=MAX_P;i++)
    if(prime[i])  
      for(int j=i;i*j<=MAX_P;j++)
	prime[i*j]=false;
}


bool calc(){
  int m,p;
  vector<int> vec;
  
  scanf("%d%d",&m,&p);
  if(m==-1) return false;
  
  for(int i=m;i<m+1000;i++){
    for(int j=i;j<m+1000;j++){
      if(prime[i] && prime[j])
      vec.push_back(i+j);
    }
  }

  sort(vec.begin(),vec.end());
  
  printf("%d\n",vec[p-1]);
  
  
  return true;
}
  
int main(){

  Eratosthenes();
  
  while(calc());
  
  return 0;
}

C++: /*
 */
#include <iostream>
#include <algorithm>

using namespace std;

// init
// fe[u
int prime[9692]; // 9,592(100,000 ÈºÌfÌ) + 100(iP ÌÅål)
int primeN;

// input
int iN; // 0 <= N <= 100,000
int iP; // 1 <= P <= 100

// manage
int iM[5050]; // 1 ~ 100 ÜÅÌa

// ú»
void init() {
  int i, j;
  bool isPrime[101198]; // 9692 ÔÚÌf = 101197

  primeN = 0;
  fill(isPrime, isPrime + 101198, true);
  isPrime[0] = isPrime[1] = false;

  for (i = 2; i < 101198; i++) {
    if (isPrime[i]) {
      prime[primeN++] = i;
      for (j = 2; i * j < 101198; j++) {
        isPrime[i * j] = false;
      }
    }
  }
}

// üÍ
bool input() {
  scanf("%d %d", &iN, &iP);
  if (iN == -1 && iP == -1) return false;
  return true;
}

// 
void manage(){
  int i, j, k;
  int cnt = 0;
  int tmp;

  // iN æèå«¢f©ç 2 ÂðIñÅaðÆé
  for (i = 0; i < primeN; i++) {
    if (prime[i] > iN) break;
  }
  for (j = 0; j < iP; j++) {
    for (k = j; k < iP; k++) {
      iM[cnt++] = prime[i + j] + prime[i + k];
    }
  }

  // ou\[g(æª©çiPÔÜÅ\[g)
  for (j = 0; j < iP; j++) {
    for (k = cnt - 1; k > j; k--) {
      if (iM[k - 1] > iM[k]) {
        tmp = iM[k];
        iM[k] = iM[k - 1];
        iM[k - 1] = tmp;
      }
    }
  }
}

// oÍ
void output(){
  printf("%d\n", iM[iP - 1]);
}

// mizoSâ¤Ê
int main() {
  init(); // ú»
  while(1) {
    if (!input()) break; // üÍ + I¹»è
    manage(); // 
    output(); // oÍ
  }
  return 0;
}