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 <cstdlib>
#include <cmath>
#include <climits>
#include <cfloat>
#include <map>
#include <utility>
#include <set>
#include <iostream>
#include <memory>
#include <string>
#include <vector>
#include <algorithm>
#include <functional>
#include <sstream>
#include <deque>
#include <complex>
#include <stack>
#include <queue>
#include <cstdio>
#include <cctype>
#include <cstring>
#include <ctime>
#include <iterator>
#include <bitset>
#include <numeric>
#include <list>
#include <iomanip>

#if __cplusplus >= 201103L
#include <array>
#include <tuple>
#include <initializer_list>
#include <unordered_set>
#include <unordered_map>
#include <forward_list>

#define cauto const auto&
#else

#endif

using namespace std;


namespace{
typedef long long LL;
typedef pair<int,int> pii;
typedef pair<LL,LL> pll;

typedef vector<int> vint;
typedef vector<vector<int> > vvint;
typedef vector<long long> vll, vLL;
typedef vector<vector<long long> > vvll, vvLL;

#define VV(T) vector<vector< T > >

template <class T>
void initvv(vector<vector<T> > &v, int a, int b, const T &t = T()){
    v.assign(a, vector<T>(b, t));
}

template <class F, class T>
void convert(const F &f, T &t){
    stringstream ss;
    ss << f;
    ss >> t;
}


#define reep(i,a,b) for(int i=(a);i<(b);++i)
#define rep(i,n) reep((i),0,(n))
#define ALL(v) (v).begin(),(v).end()
#define PB push_back
#define F first
#define S second
#define mkp make_pair
#define RALL(v) (v).rbegin(),(v).rend()


#define MOD 1000000007LL
#define EPS 1e-8
static const int INF=1<<24;

const int maxn=17000000;
int f[maxn],prime[maxn];
 
int GenP() {
	int i,tot,p,tmp;
	tot=0;
	for (i=2;i<maxn;++i) f[i]=i;
	for (p=2;p<maxn;++p)
	{
		if (f[p]==p) prime[tot++]=p;
		for (i=0;i<tot && prime[i]<=f[p];++i)
		{
			tmp=prime[i]*p;
			if (tmp<maxn) 
				f[tmp]=prime[i];
			else
				break;
		}
	}
}


void mainmain(){
	// rep(i,10){
		// cout<<prime[i+1000000-10]<<endl;
	// }
	GenP();
	set<int> se;
	rep(i,1000003){
		se.insert(prime[i]);
	}
	int n;
	while(cin>>n,n){
		
		auto it1=se.lower_bound(n);
		auto it2=se.upper_bound(n);
		if(*it1!=*it2){
			cout<<0<<endl;
			continue;
		}
		it2--;
		if(*it1==*it2){
			cout<<0<<endl;
		}
		else{
			cout<<*it1-*it2<<endl;
		}
	}
}



}
main() try{
    mainmain();
}
catch(...){
}

C++: #include <algorithm>
#include <bitset>
#include <complex>
#include <deque>
#include <exception>
#include <fstream>
#include <functional>
#include <iomanip>
#include <ios>
#include <iosfwd>
#include <iostream>
#include <istream>
#include <iterator>
#include <limits>
#include <list>
#include <locale>
#include <map>
#include <memory>
#include <new>
#include <numeric>
#include <ostream>
#include <queue>
#include <set>
#include <sstream>
#include <stack>
#include <stdexcept>
#include <streambuf>
#include <string>
#include <typeinfo>
#include <utility>
#include <valarray>
#include <vector>
#include <climits>
#include <cstring>
#include <cassert>


using namespace std;
#define SZ(x) (int)(x.size())
#define REP(i, n) for(int i=0;i<n;++i)
#define FOR(i, a, b) for(int i=a;i<b;++i)
#define RREP(i, n) for(int i=n-1;i>=0;--i)
#define RFOR(i, a, b) for(int i=b-1;i>=a;--i)

#define all(x) begin(x),end(x)
#define dump(x) cerr<<#x<<" = "<<(x)<<endl
#define debug(x) cerr<<#x<<" = "<<(x)<<" (L"<<__LINE__<<")"<< endl;

using ll = long long;
using vi = vector<int>;
using vvi = vector<vi>;
using vll = vector<ll>;
using vvll = vector<vll>;
using P = pair<int, int>;
const double eps = 1e-8;
const ll MOD = 1000000007;
const int INF = INT_MAX / 2;
const ll LINF = LLONG_MAX / 2;

template<class T>
bool chmax(T &a, const T &b) {
    if (a < b) {
        a = b;
        return true;
    }
    return false;
}

template<class T>
bool chmin(T &a, const T &b) {
    if (b < a) {
        a = b;
        return true;
    }
    return false;
}


template<typename T1, typename T2>
ostream &operator<<(ostream &os, const pair<T1, T2> p) {
    os << p.first << ":" << p.second;
    return os;
}

template<class T>
ostream &operator<<(ostream &os, const vector<T> &v) {
    os << "[";
    REP(i, SZ(v)) {
        if (i) os << ", ";
        os << v[i];
    }
    return os << "]";
}

// edit
vector<int> primes;

bool solve(int N) {
    if(N == 0) return false;
    
    int x = *(upper_bound(all(primes), N) - 1);
    int y = *lower_bound(all(primes), N);
    int ans = y - x;
    cout << ans << endl;

    return true;
}


int main() {
    cin.tie(0);
    ios::sync_with_stdio(false);
    cout << fixed << setprecision(10);
//    std::ifstream in("input.txt");
//    std::cin.rdbuf(in.rdbuf());

//    solve();
    vector<bool> is_prime(1299710, true);
    for(int i = 2; i < is_prime.size(); ++i){
        if(is_prime[i]){
            for(int j = i * 2; j < is_prime.size(); j += i){
                is_prime[j] = false;
            }
        }
    }

    for(int i = 1; i < is_prime.size(); ++i){
        if(is_prime[i]){
            primes.push_back(i);
        }
    }

    
    while(true){
        int N;
        cin >> N;
        if(!solve(N)){
            break;
        }
    }


    return 0;
}