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<cassert>
#include<ctime>
#include<climits>
#include<queue>
#include<fstream>
#include<deque>
#include<iomanip>
#include<list>
#include<stack>
#include<sstream>
#include<string>
#include<cctype>
#include<cstdio>
#include<vector>
#include<cmath>
#include<cstdlib>
#include<algorithm>
#include<cstring>
#include<map>
#include<set>
#define REP(i,s,n) for(int i=s;i<n;i++)
#define rep(i,n) REP(i,0,n)
#define EPS 1e-10
#define F first
#define S second
#define all(n) n.begin(),n.end()
#define pb push_back
#define insert(a,b,c,d) PP(P(a,b),P(c,d))
#define INF (1<<27)
using namespace std;
typedef long long ll;
typedef unsigned long long ull;
typedef pair<int,int> P;
typedef pair<P,P> PP;
const int dx[4] = {0,1,0,-1};
const int dy[4] = {1,0,-1,0};
const int LIM = 1900000;
bool prime[LIM+2];
vector<int> primes;
void eratos(){
 
  int p=0;
  for(int i=0;i<=LIM;i++)prime[i] = true;
  prime[0] = prime[1] = false;
  for(int i=2;i<=LIM;i++){
    if(prime[i]){
      for(int j=2*i;j<=LIM;j+=i)prime[j] = false;
    }
  }
  
  REP(i,2,LIM){
    if(prime[i]){
      primes.pb(i);
    }
  }
  // cout << primes.size() << endl;///////////////////////////////
}
 
int b_search(int key){
  if(key == 0)return 0;
  if(key == 1)return 0;
  int l=0,r=primes.size()-1;
 
  while(l <= r){
    int mid = (l+r)/2;
    if(key <= primes[mid] && primes[mid-1] < key)return mid;
    if(key < primes[mid])r = mid;
    else if(primes[mid] < key)l =mid+1;
    //cout << "r = "<< r << ", l = " << l << endl;
  }
  assert(false);
}
 
int main(){
  clock_t start,end;
  start = clock();
  eratos();
  end = clock();
 
  //cout << setiosflags(ios::fixed) << setprecision(10) << "eratos time = " <<(double)(end-start)/CLOCKS_PER_SEC << endl;
  int N,P;
  while(true){
    
    cin >> N >> P;
    if(N == -1 && P == -1)break;
    
    int index = b_search(N);
    if(prime[N])index++;
    //cout << "index = " << index << " " << primes[index] << endl;    
    vector<int> Set;
    for(int i=index;i<=index+P;i++){
      for(int j=i;j<=index+P;j++){
    Set.pb(primes[i]+primes[j]);
      }
    }
    sort(all(Set));
    cout << Set[P-1] << endl;
  }
return 0;
}

C++: #include <cstdio>
#include <iostream>
#include <sstream>
#include <fstream>
#include <iomanip>
#include <algorithm>
#include <cmath>
#include <string>
#include <vector>
#include <list>
#include <queue>
#include <stack>
#include <set>
#include <map>
#include <bitset>
#include <numeric>
#include <climits>
#include <cfloat>
using namespace std;

void findPrime(int N, vector<bool>& isPrime)
{
    isPrime.assign(N+1, true);
    isPrime[0] = isPrime[1] = false;
    for(int i=2; i*i<=N; i++){
        if(isPrime[i]){
            for(int j=i; i*j<=N; j++){
                isPrime[i*j] = false;
            }
        }
    }
}

int main()
{
    vector<bool> prime;
    findPrime(200000, prime);

    for(;;){
        int n, p;
        cin >> n >> p;
        if(n == -1)
            return 0;

        vector<int> a;
        for(int i=n+1; ; ++i){
            if(prime[i]){
                a.push_back(i);
                if(a.size() == p)
                    break;
            }
        }

        vector<int> b;
        for(int i=0; i<p; ++i){
            for(int j=i; j<p; ++j){
                b.push_back(a[i] + a[j]);
            }
        }
        sort(b.begin(), b.end());

        cout << b[p-1] << endl;
    }
}

C++: #include <algorithm>
#include <iostream>
#include <vector>
using namespace std;
int main(){
  int M = 1000000;
  bool P[M];
  for(int i = 0; i < M; ++i) P[i] = true;
  P[0] = false;
  P[1] = false;

  for(int i = 2; i*i < M; ++i){
    for(int j = i; i*j < M; ++j){
      P[i*j] = false;
    }
  }

  
  int N, p;
  while(cin >> N >> p, N != -1){
    int c = 0;
    vector<int> V;
    while(c < p){
      for(int i = N+1; i < M; ++i){
	if(P[i]){
	  V.push_back(i);
	  ++c;
	}
      }
    }
    vector<int> ans;
    for(int i = 0; i < p; ++i){
      for(int j = i; j < p; ++j){
	ans.push_back(V[i]+V[j]);
      }
    }
    sort(ans.begin(), ans.end());
    cout << ans[p-1] << endl;
  }
  return 0;
}