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 <cmath>
#include <vector>
#include <string>
#include <map>
#include <algorithm>
#include <tuple>
#include <set>
#include <stack>
#include <queue>


using namespace std;
using ll = long long;
using ull = unsigned long long;
using vi = vector<int>;
using vvi = vector<vi>;
using vl = vector<ll>;
using pii = pair<int, int>;
using vpii = vector<pii>;
using pci = pair<char, int>;
using vpci = vector<pci>;
#define out(S) cout<<(S)<<endl;
#define REP(i,b) for(size_t i=0;i<(b);i++)
#define rREP(i,b) for(int i=(b)-1;i>=0;i--)
#define FOR(i,a,b) for(size_t i=(a);i<(b);i++)
#define rFOR(i,a,b) for(int i=(a)-1;i>=(b);i--)
#define Foreach(item,collection) for(auto item:collection)
#define mod(i) ((i) % (ll)(1e9 + 7))
ll modpow(ll i, ll j) { ll tmp = 1; while (j) { if (j % 2)tmp = mod(tmp*i); i = mod(i*i); j /= 2; }return tmp; }
#define divmod(a,b) (mod(a * modpow((ll)b,(ll)(1e9 + 5))))
#define Yes out("Yes")
#define No out("No")
#define NO out("NO")
#define YES out("YES")
#define INF 1e10;
#define ShowAll(collection) for(auto i:collection){out(i);}
#define IfOut(condition,text) if((condition)){out(text);return 0;}
#define IfeOut(condition,itext,etext) if(condition){out(itext);return 0;}else{out(etext);return 0;}
#define Select(collection,condition,result) Foreach(i,collection){result+=condition;}
#define GetAuto(_N,vect) {cin>>_N;vect.resize(_N);REP(i,_N){cin>>vect[i];}}
#define beginend(vec) (vec).begin(),(vec).end()
#define pb(item) push_back(item)
#define removeAll(vec,deleg) auto itr=remove_if(beginend(vec),deleg);vec.erase(itr,vec.end());
void removeAt(string& s, int index) { s.erase(index, 1); }
template<typename T> void removeAt(vector<T>& v, int index) { v.erase(v.begin() + index); }
ll manhattanDistance(ll x1, ll y1, ll x2, ll y2) { return (abs(x2 - x1) + abs(y2 - y1)); }
vector<ll> sieveOfEratosthenes(int max) { ll prime; double sqrtmax = sqrt(max); vector<ll> primeVec, searchVec; FOR(i, 2, max + 1) searchVec.push_back(i); do { prime = searchVec[0]; primeVec.push_back(prime); auto itr = remove_if(beginend(searchVec), [=](ll x)->bool {return x%prime == 0; }); searchVec.erase(itr, searchVec.end()); } while (prime < sqrtmax); primeVec.reserve(primeVec.size() + searchVec.size()); primeVec.insert(primeVec.end(), beginend(searchVec)); return primeVec; }
bool isPrime(ll x) { if (x == 2) return true; if (x < 2 || x % 2 == 0) return false; ll i = 3; while (i <= sqrt(x)) { if (x%i == 0) return false; i += 2; }return true; }
vector<ll> rangePrimes(int start, int cnt) {
	vl rt;
	int n = start+1;
	if (n <= 1) n = 2;
	if (n == 2) {
		rt.pb(2); n++;
	}
	if (n % 2 == 0) n++;
	while (cnt) {
		if (isPrime(n)) {
			rt.pb(n); cnt--;
		}
		n += 2;
	}
	return rt;
}


int N, P;

int main() {
	while (cin>>N>>P,(N!=-1||P!=-1))
	{
		auto box = rangePrimes(N, P);
		int cnt = 0;
		bool f = true;
		
		vl sss;
		int bsize = box.size();
		FOR(i, 0, bsize) {
			FOR(x, i, bsize) {
				sss.pb(box[i] + box[x]);
			}
		}
		sort(beginend(sss));

		out(sss[P-1]);
	}

	return 0;
}

C++: #include <iostream>
#include <algorithm>
#include <functional>
#include <vector>
#include <set>
#include <queue>
#include <cstring>
#include <cstdio>
#include <cmath>

using namespace std;

#define EPS	1.0e-10
#define rep(idx, max)	for(int idx = 0, idx##Max = max; idx < idx##Max; idx ++)
#define rrep(idx, min, max)	for(int idx = min, idx##Max = max; idx < idx##Max; idx ++)
#define erep(idx, min, max)	for(int idx = min, idx##Max = max; idx <= idx##Max; idx ++)

#define rev(idx, max)	for(int idx = max - 1; idx >= 0; idx --)

#define foreach(it, o)	for(auto it = (o).begin(), it##End = (o).end(); it != it##End; it ++)

#define SORT_INCORDER(v)	std::sort((v).begin(), (v).end())
#define SORT_DECORDER(v)	std::sort((v).begin(), (v).end(), std::isgreater<int>())


typedef unsigned int UINT;
typedef std::vector<int>	VEC;

/*
* ?´???°?????????
* @param (dst) ?´???°?????????????????????
* @param (n) ?????§???
*/
void	GetPrime(VEC &dst, int n)
{
	VEC bIsPrime(n);

	rep(i, n) { bIsPrime[i] = (i & 0x01); }
	bIsPrime[1] = false;
	bIsPrime[2] = true;
	for (int i = 2; i * i < n; i++)
	{
		if (!bIsPrime[i]) { continue; }
		for (int j = i * i; j < n; j += i)	{	bIsPrime[j] = false;	}
	}

	dst.clear();
	rep(i, n) { if (bIsPrime[i]) { dst.push_back(i); } }
}

#define MAX_TERM	100
#define MAX_NUMS	2e+5

int main(int nArgs, char **lplpszArgs)
{
	VEC	primes;
	vector<int>	sums;
	
	int	bottomNum, term;

	::GetPrime(primes, MAX_NUMS);

	while (cin >> bottomNum >> term, bottomNum != -1)
	{
		int	idxStart = 0;	// bottomNum??\???????´???°?????????????????????
		rep(i, MAX_NUMS)
		{
			if (bottomNum < primes[i]) { idxStart = i; break; }
		}
		erep(i, idxStart, idxStart + term - 1)
		{
			erep(j, i, idxStart + term - 1)
			{
				sums.push_back(primes[i] + primes[j]);
			}
		}
		
		SORT_INCORDER(sums);
		//rep(i, term){	::printf("[%d]", sums[i]);	}	::puts("");

		::printf("%d\n", sums[term - 1]);
		sums.clear();
	}
}