Go: package main

import "fmt"

func main() {
	var i int64
	var list = []int64{13, 19, 109, 199, 829, 1489, 1879, 2089, 3259, 3469, 5659, 9439, 13009, 15649, 15739, 16069, 18049, 18919, 19429, 21019, 22279, 25309, 31729, 34849, 43789, 51349, 55339, 62989, 67219, 69499, 72229, 77269, 79699, 81049, 82729, 88819, 97849, 99139, 101119, 109849, 116539, 119299, 122209, 135469, 144169, 157279, 165709, 166849, 171169, 187639, 194869, 195739, 201499, 201829, 217369, 225349, 240049, 243709, 247609, 247999, 257869, 260419, 266689, 268819, 276049, 284749, 285289, 294319, 295879, 299479, 300499, 301999, 326149, 334429, 340939, 346399, 347989, 354259, 358909, 361219, 375259, 388699, 389569, 392269, 394819, 397549, 397759, 402139, 402769, 412039, 419059, 420859, 427249, 442579, 444349, 452539, 463459, 465169, 467479, 470089, 477019, 490579, 495619, 500239, 510619, 518809, 536449, 536779, 539509, 549169, 559219, 563419, 570049, 572659, 585919, 594829, 597679, 607309, 622249, 626629, 632089, 632329, 633469, 633799, 654169, 657499, 661099, 663589, 664669, 666439, 680299, 681259, 691729, 705169, 715159, 734479, 736369, 739399, 768199, 773029, 795799, 803449, 814069, 822169, 823729, 829729, 833719, 837079, 845989, 854929, 855739, 857959, 875269, 876019, 881479, 889879, 907399, 930079, 938059, 946669, 954979, 958549, 959479, 976309, 978079, 983449, 1002349, 1003369, 1006309, 1006339, 1008859, 1015369, 1022389, 1022509, 1023949, 1027759, 1043599, 1063969, 1065019, 1068259, 1068709, 1091269, 1093069, 1093999, 1100839, 1117609, 1117819, 1118869, 1120549, 1121839, 1122139, 1126669, 1137889, 1146799, 1155619, 1156039, 1157839, 1163719, 1167709, 1168249, 1170139, 1172029, 1172539, 1173589, 1182289, 1210879, 1228399, 1230379, 1233439, 1246249, 1246369, 1257079, 1272289, 1285519, 1298119, 1322179, 1322599, 1324579, 1329709, 1337269, 1339909, 1340329, 1351249, 1352209, 1358809, 1360789, 1368469, 1381279, 1400809, 1402369, 1410979, 1440589, 1447009, 1451839, 1461409, 1468639, 1508629, 1514329, 1524079, 1525969, 1540969, 1571749, 1573549, 1573939, 1577299, 1584439, 1588759, 1592869, 1603339, 1615849, 1616809, 1621729, 1627609, 1631059, 1653109, 1659109, 1670569, 1678759, 1681879, 1685449, 1691869, 1711819, 1718869, 1727779, 1747729, 1748479, 1749499, 1755829, 1764229, 1780489, 1791739, 1797379, 1798639, 1802659, 1819849, 1830349, 1833529, 1834039, 1837399, 1857679, 1890529, 1902619, 1904479, 1910269, 1912069, 1912459, 1915939, 1917739, 1925389, 1943659, 1954159, 1954369, 1959319, 1979149, 1979899, 2002339, 2007619, 2007919, 2016409, 2020729, 2050339, 2062009, 2063779, 2065579, 2075839, 2079199, 2083519, 2084449, 2087389, 2107669, 2116579, 2136139, 2141809, 2143489, 2144509, 2157739, 2159239, 2168659, 2176639, 2177509, 2193889, 2202799, 2203969, 2223679, 2225059, 2246149, 2248249, 2256349, 2262979, 2266639, 2289649, 2290039, 2294059, 2303599, 2340259, 2342779, 2362279, 2373409, 2376169, 2381089, 2403889, 2413429, 2418679, 2423419, 2443789, 2458669, 2470339, 2478529, 2479669, 2499949, 2508049, 2535109, 2541949, 2546239, 2552119, 2552659, 2561269, 2564329, 2576599, 2594959, 2594989, 2596669, 2604739, 2614069, 2635489, 2668249, 2673529, 2674549, 2679499, 2683789, 2696929, 2704909, 2712379, 2715289, 2728549, 2731699, 2731909, 2759299, 2761729, 2764129, 2790259, 2805169, 2822719, 2839939, 2840269, 2846869, 2849689, 2875339, 2879299, 2893489, 2918569, 2927599, 2927809, 2952799, 2954689, 2989039, 2990839, 3047419, 3058879, 3062209, 3063499, 3066829, 3076399, 3089329, 3092569, 3101479, 3103279, 3129619, 3153589, 3157579, 3171739, 3187609, 3200209, 3207439, 3208819, 3215749, 3224869, 3243349, 3277699, 3295549, 3308959, 3326629, 3328909, 3340879, 3366829, 3371449, 3374479, 3378049, 3400219, 3416059, 3436249, 3437719, 3467539, 3479899, 3512059, 3512239, 3512989, 3513079, 3514549, 3518329, 3550699, 3584929, 3586909, 3593509, 3621469, 3652939, 3669769, 3690529, 3713449, 3727729, 3735169, 3741169, 3798079, 3817819, 3837139, 3837859, 3849829, 3854119, 3894049, 3919219, 3919

C++: #include<iostream>

bool isPrime(int a){
	bool ans = true;
	if(a%2 == 0 || a%3 == 0 || a%5 ==0 || a%7 == 0)
		ans = false;
	else{
		for(int i = 11; i*i <= a; i++){
			if(a%i == 0){
				ans = false;
				break;
			}
		}
	}
	return ans;
}

int n;

int main(){
	while(true){
		std::cin >> n;
		if(!n)
			return 0;
		if(n%2 == 0)
			n--;
		for(; n>12; n-=2){
			if(n==13){
				std::cout << 13 << std::endl;
			}else if(isPrime(n)){
				if(isPrime(n-2)){
					if(isPrime(n-6)){
						if(isPrime(n-8)){
							std::cout << n << std::endl;
							break;
						}
					}
				}
			}
		}
	}
}

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

using namespace std;

const int N = 10000000;

int main() {
  int n;
  vector<bool> is_prime(N + 1, true);
  is_prime[0] = is_prime[1] = false;
  for (int i = 2; i <= N; ++i) {
    for (int j = 2; i * j <= N; ++j) {
      is_prime[i * j] = false;
    }
  }
  while (true) {
    cin >> n;
    if (n == 0) {
      return 0;
    }
    for (int k = n; true; --k) {
      if (k % 6 == 1 and is_prime[k] and is_prime[k - 2]
          and is_prime[k - 6] and is_prime[k - 8]) {
        cout << k << endl;
        break;
      }
    }
  }
}

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

int prime[1000000];
int prime4[1000000];
int GetPrime( int n );
int GetPrime4( int pcnt );

int main(){
	int i,n;
	int pcnt,p4cnt;
	pcnt = GetPrime( 11000000 );
	//cout << pcnt << endl;
	p4cnt = GetPrime4( pcnt );

	while( cin >> n ){
		if( n == 0 )
			break;
		for( i = 0;prime4[i] <= n;i++ );
		cout << prime4[i-1] << endl;
	}
	return 0;
}

int GetPrime(int n){
	int i,j;

	int cnt = 0;
	for( i = 2;i <= n;i++ ){
		for( j = 0;j < cnt;j++ ){
			if( i%prime[j] == 0 )
				break;
			if( prime[j] * prime[j] > i ){
				j = cnt;
				break;
			}
		}
		if( j == cnt ){
			prime[cnt] = i;
			cnt++;
		}
	}
	return cnt;
}

int GetPrime4( int pcnt ){
	int i;
	int cnt = 0;
	for( i = 0;i < pcnt-3;i++ ){
		if( prime[i+1] - prime[i] == 2 && prime[i+2] - prime[i+1] == 4 && prime[i+3] - prime[i+2] == 2 ){
			prime4[cnt] = prime[i+3];
			cnt++;
		}
	}
	return cnt;
}

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

bool prime[10000000];
void primenum(void)
{
	for (int i = 0; i < 10000000; i++){
		prime[i] = true;
	}
	prime[1] = prime[0] = false;
	for (int i = 2; i*i < 10000000; i++){
		for (int j = i * 2; j < 10000000; j += i){
			prime[j] = false;
		}
	}
}

int main(void)
{
	int n;
	primenum();
	while (scanf("%d", &n), n){
		for (int i = n; 0 <= i; i--){
			if (prime[i]&&prime[i-2]&&prime[i-6]&&prime[i-8]){
				printf("%d\n", i);
				break;
			}
		}
	}
	return 0;
}

C++: 
#include<cstdio>
#include<iostream>
#include<algorithm>
#include<string>
#include<queue>
#include<vector>
#include<functional>
#include<cmath>
#include<map>
#include<stack>
#include<set>
#include<numeric>
#include<limits>
#include<iterator>

#define all(x) (x).begin(),(x).end()
#define rall(x) (x).rbegin(),(x).rend()
#define rep(i,n) for(int i=0; i<n; i++)
#define INF (1<<20)

using namespace std;
typedef long long ll;
typedef pair<int, int> pi;
typedef pair<ll, ll> pl;
typedef pair<ll, char> plc;

ll n;
ll arr[10000010],ans[10000010];

void Eratosthenes() {
	for (ll i = 0; i <= 10000010; i++)arr[i] = 1;
	
	for (ll i = 2; i <= sqrt(10000010); i++) {
		if (arr[i]) {
			for (ll j = 0; i*(j + 2) <= 10000010; j++)
				arr[i*(j + 2)] = 0;
		}
	}

	ll pre_prime = 0;
	for (ll i = 13; i <= 10000010; i++) {
		if (arr[i] && arr[i-2] && arr[i-6] && arr[i-8]) {
			ans[i] = i;
			pre_prime = i;
		}
		else ans[i] = pre_prime;
	}
}

C++: #include <cstdio>

char prmt[10000005]={};

int rest[10000005]={};

int main(void){
int n,i,j;
prmt[0]=-1;
prmt[1]=-1;
for(i=2;i<=10000000;i++){
	if(prmt[i]==-1)continue;
	prmt[i]=1;
	j=i*2;
	while(j<=10000000){
		prmt[j]=-1;
		j+=i;
	}
}
j=13;
for(i=13;i<=10000000;i++){
	if(prmt[i]==1 && prmt[i-2]==1 && prmt[i-6]==1 && prmt[i-8]==1)j=i;
	rest[i]=j;
}
while(1){
scanf("%d",&n);
if(n==0)break;
printf("%d\n",rest[n]);
}
return 0;
}