Go: package main

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

func main() {
	io := NewIo()
	defer io.Flush()
	isPrime := Sieve(1299709)
	for {
		k := io.NextInt()
		if k == 0 {
			break
		}
		ans := 0
		if !isPrime[k] {
			p, n := k-1, k+1
			for !isPrime[p] || !isPrime[n] {
				if !isPrime[p] {
					p--
				}
				if !isPrime[n] {
					n++
				}
			}
			ans = n - p
		}
		io.Println(ans)
	}
}

func Sieve(x int) []bool {
	flags := make([]bool, x+1)
	for i := 2; i <= x; i++ {
		flags[i] = true
	}
	for i := 2; i <= int(math.Sqrt(float64(x))); i++ {
		if !flags[i] {
			continue
		}
		for j := i * i; j <= x; j += i {
			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 := make([]interface{}, 0, len(a))
	for _, x := range a {
		b = append(b, x)
	}
	io.Println(b...)
}

func (io *Io) PrintStringSlice(a []string) {
	b := make([]interface{}, 0, len(a))
	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<string>
#include<algorithm>
#include<map>
#include<set>
#include<utility>
#include<vector>
#include<cmath>
#include<cstdio>
#define loop(i,a,b) for(int i=a;i<b;i++) 
#define rep(i,a) loop(i,0,a)
#define pb push_back
#define mp make_pair
#define it ::iterator
#define all(in) in.begin(),in.end()
const double PI=acos(-1);
const double ESP=1e-10;
using namespace std;
int main(){
  #define MAX 2000000
  bool sosu[MAX]={0};
  for(int i=4;i<MAX;i+=2)sosu[i]=true;
  for(int i=3;i<MAX;i+=2){
	if(sosu[i])continue;
	for(int j=i*3;j<MAX;j+=i*2)sosu[j]=true;
  }
  int n;
  while(cin>>n,n){
	int co=0;
	for(int i=n;i<MAX;i++){
	  if(!sosu[i])break;
	  co++;
	}
	for(int i=n;i>1;i--){
	  if(!sosu[i])break;
	  co++;
	}
	cout<<co<<endl;
  }
}

C++: #include<iostream>
using namespace std;
int isPrime(int n){
  for(int i=2;i*i<=n;i++) if(n%i==0) return 0;
  return 1;
}
int main(){
  int i,j,dp[1500000]={},c;
  i=2;
  
  while(i<1500000){
    c=1;
    dp[i]=0;
    while(isPrime(i+c)==0){
      c++;
    }
    if(i+c<1500000){
      for(j=i+1;j<i+c;j++) dp[j]=c;
      dp[i+c]=0;
    }
    i+=c;
  }
  int n;
  cin >> n;
  while(n!=0){
    cout << dp[n] << endl;
    cin >> n;
  }
  return 0;
}

C++: #include <bits/stdc++.h>
#define syosu(x) fixed<<setprecision(x)
using namespace std;
typedef long long ll;
typedef unsigned int uint;
typedef unsigned long long ull;
typedef pair<int,int> P;
typedef pair<double,double> pdd;
typedef pair<ll,ll> pll;
typedef vector<int> vi;
typedef vector<vi> vvi;
typedef vector<double> vd;
typedef vector<vd> vvd;
typedef vector<ll> vl;
typedef vector<vl> vvl;
typedef vector<string> vs;
typedef vector<P> vp;
typedef vector<vp> vvp;
typedef vector<pll> vpll;
typedef pair<ll,pll> pip;
typedef vector<pip> vip;
const int inf=1<<30;
const ll INF=1ll<<60;
const double pi=acos(-1);
const double eps=1e-11;
const ll mod=1e9+7;
const int dx[4]={-1,0,1,0},dy[4]={0,-1,0,1};
const int DX[8]={-1,0,1,0,-1,-1,1,1},DY[8]={0,-1,0,1,-1,1,-1,1};

const int NPrime=3000000;
bool prime[NPrime+1];
vi EnumeratePrime(){
	fill(prime+2,prime+NPrime+1,true);
	vi a;
	for(int i=2;i<=NPrime;i++) if(prime[i]){
		a.emplace_back(i);
		for(int j=2*i;j<=NPrime;j+=i) prime[j]=0;
	}
	return a;
}

int n;

int main(){
	EnumeratePrime();
	while(1){
		cin>>n;
		if(n==0) break;
		if(prime[n]) cout<<0<<endl;
		else{
			int t=2;
			for(int i=n-1;!prime[i];i--) t++;
			for(int i=n+1;!prime[i];i++) t++;
			cout<<t<<endl;
		}
	}
}

C++: #include <algorithm>
#include <cmath>
#include <climits>
#include <cstdio>
#include <cstdlib>
#include <cstring>
#include <fstream>
#include <iostream>
#include <list>
#include <map>
#include <queue>
#include <set>
#include <sstream>
#include <stack>
#include <string>
#include <vector>
#include <cassert>
#include <functional>

using namespace std;

#define LOG(...) printf(__VA_ARGS__)
//#define LOG(...)
#define FOR(i,a,b) for(int i=(int)(a);i<(int)(b);++i)
#define REP(i,n) for(int i=0;i<(int)(n);++i)
#define ALL(a) (a).begin(),(a).end()
#define RALL(a) (a).rbegin(),(a).rend()
#define EXIST(s,e) ((s).find(e)!=(s).end())
#define SORT(c) sort((c).begin(),(c).end())
#define RSORT(c) sort((c).rbegin(),(c).rend())
#define CLR(a) memset((a), 0 ,sizeof(a))

typedef long long ll;
typedef unsigned long long ull;
typedef vector<bool> vb;
typedef vector<int> vi;
typedef vector<ll> vll;
typedef vector<vb> vvb;
typedef vector<vi> vvi;
typedef vector<vll> vvll;
typedef pair<int, int> pii;
typedef pair<ll, ll> pll;

const int dx[] = { -1, 0, 1, 0 }; const int dy[] = { 0, 1, 0, -1 };

struct UnionFind {
	vector<int> v;
	UnionFind(int n) : v(n)  { for (int i = 0; i < n; i++) v[i] = i; }
	int  find(int x)         { return v[x] == x ? x : v[x] = find(v[x]); }
	void unite(int x, int y) { v[find(x)] = find(y); }
};
bool b[1299710];
int main() {
	vi pri;
	FOR(i,2, 1299710) {
		if (!b[i]) {
			pri.push_back(i);
			for (int j = i * 2;j < 1299710;j += i) {
				b[j] = true;
			}
		}
	}
	int n;
	while (cin>>n,n) {
		int l = 0;
		int r = 99999;
		while (abs(l-r)>1) {
			if (pri[(l + r) / 2] > n)
				r = (l + r) / 2;
			else
				l = (l + r) / 2;
		}
		if (pri[l] == n || pri[r] == n)
			cout << 0 << endl;
		else
			cout << pri[r] - pri[l] << endl;
	}
}

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

bool so(int n){
  int cnt=0;
  for(int i=2;i*i<=n;i++){
    if(n%i==0){
      cnt++;
      break;
    }
  }
  if(cnt==0)return true;
  else return false;
}

int main(){
  int n;
  while(cin>>n,n){
    int cnt=0;
    if(!so(n)){
      for(int i=n+1;;i++){
        if(so(i))break;
        else cnt++;
      }
      for(int i=n-1;;i--){
        if(so(i))break;
        else cnt++;
      }
      cnt+=2;
    }
    cout<<cnt<<endl;
  }
  return 0;
}