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 <bits/stdc++.h>

using namespace std;

typedef vector<int> vi;
typedef vector<vi> vii;
typedef vector<string> vs;
typedef vector<bool> vb;
typedef vector<vb> vbb;
typedef pair<int, int> pii;
typedef long long ll;
typedef unsigned long long ull;

#define all(a)  (a).begin(),(a).end()
#define rall(a) (a).rbegin(), (a).rend()
#define pb push_back
#define mp make_pair

#define loop(i,a,b) for(int i=(a);i<ull(b);++i)
#define rep(i,n) loop(i,0,n)

const double eps = 1e-10;
const double pi  = acos(-1.0);
const double inf = (int)1e8;

#define MAX 2000010

bool e[MAX];

void erato(){
    fill(e, e+MAX, true);
    e[0] = e[1] = false;

    for(int i=0; i < MAX; i++){
        if(!e[i]) continue;
        for(int j=i+i; j < MAX; j += i){
            e[j] = false;
        }
    }
}

int main(){
    erato();
    int n;
    while(cin >> n, n){
        int ret = 0;
        if(e[n]){cout << 0 << endl; continue;}
        while(!e[n]) n--;
        for(int i=n+1; !e[i]; i++)ret++;
        cout << ret+1 << endl;
    }
}

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

#define rep(i, n) for(int i = 0; i < n; i ++)
#define COUNT(i, n) for(int i = 1; i <= n; i ++)
#define ITER(c) __typeof((c).begin())
#define each(c, it) for(ITER(c) it =(c).begin(); it!=(c).end(); it++)
#define ALL(c) c.begin(), c.end()
#define mp make_pair
#define pb push_back
#define MEMSET(v, h) memset((v), h, sizeof(v))
 
using namespace std; 

typedef pair<int, int> pii;
typedef long long ll;
typedef vector<int> vi;
typedef vector<string> vs;

const int INF = 1 << 24;

int main(void){
	const int N = 1299710;
	bool isP[N];
	rep(i, N) isP[i] = true;
	for(int i = 2; i * i < N; i ++) if(isP[i]) for(int j = i; i * j < N; j ++) isP[i * j] = false;

    for(int n, r, res; cin >> n, n; cout << res << endl){
        r = n;
        if(isP[n]) res = 0;
        else{
            while(!isP[--r]);
            while(!isP[++n]);
        }
        res = n - r;
    }

    return 0;
}

C++: #include "bits/stdc++.h"
using namespace std;
typedef long long ll;
typedef pair<int, int> pii;
typedef pair<ll, ll> pll;
const int INF = 1e9;
const ll LINF = 1e18;
template<class S,class T> ostream& operator << (ostream& out,const pair<S,T>& o){ out << "(" << o.first << "," << o.second << ")"; return out; }
template<class T> ostream& operator << (ostream& out,const vector<T> V){ for(int i = 0; i < V.size(); i++){ out << V[i]; if(i!=V.size()-1) out << " ";} return out; }
template<class T> ostream& operator << (ostream& out,const vector<vector<T> > Mat){ for(int i = 0; i < Mat.size(); i++) { if(i != 0) out << endl; out << Mat[i];} return out; }
template<class S,class T> ostream& operator << (ostream& out,const map<S,T> mp){ out << "{ "; for(auto it = mp.begin(); it != mp.end(); it++){ out << it->first << ":" << it->second; if(mp.size()-1 != distance(mp.begin(),it)) out << ", "; } out << " }"; return out; }

/*
 <url:http://judge.u-aizu.ac.jp/onlinejudge/description.jsp?id=1276>
 問題文============================================================
 =================================================================
 解説=============================================================
 ================================================================
 */

const ll MAX_PRIME = 1299710;
vector<int> primes;
vector<int> is_prime(MAX_PRIME + 1,true);
void init_primes(){
    is_prime[0] = is_prime[1] = false;
    for(int i = 2; i <= MAX_PRIME;i++){
        if(is_prime[i]){
            primes.push_back(i);
            for(int j = i*2; j <= MAX_PRIME; j+=i) is_prime[j] = false;
        }
    }
}

int K;
ll solve(){
    ll res = 0;
    if(is_prime[K]) return 0;
    auto it = upper_bound(primes.begin(),primes.end(),K);
    res = *it - *(it-1);
    return res;
}
int main(void) {
    cin.tie(0); ios_base::sync_with_stdio(false);
    init_primes();
    while(cin >> K,K){
        cout << solve() << endl;
    }
    return 0;
}

C++: #include <vector>
#include <iostream>
#include <algorithm>
using namespace std;
vector<int> P;
const int pmax=1299709;
bool iscomp[pmax+1];
void init()
{
	P.reserve(100000);
	for(int i=2;i<pmax+1;i++){
		if(iscomp[i])continue;
		P.push_back(i);
		for(int j=2;i*j<pmax+1;j++)
			iscomp[i*j]=true;
	}
}
int main()
{
	init();
	while(true){
		int n;
		cin>>n;
		if(n==0)return 0;
		int pos=lower_bound(P.begin(),P.end(),n)-P.begin();
		if(P[pos]==n)cout<<0<<endl;
		else{
			cout<<P[pos]-P[pos-1]<<endl;
		}
	}
}

C++: #define _USE_MATH_DEFINES
#include "bits/stdc++.h"
using namespace std;

//#define int long long
#define DBG 1
#define dump(o) if(DBG){cerr<<#o<<" "<<o<<endl;}
#define dumpc(o) if(DBG){cerr<<#o; for(auto &e:(o))cerr<<" "<<e;cerr<<endl;}
#define rep(i,a,b) for(int i=(a);i<(b);i++)
#define rrep(i,a,b) for(int i=(b)-1;i>=(a);i--)
#define each(it,c) for(auto it=(c).begin();it!=(c).end();it++)
#define all(c) c.begin(),c.end()
const int INF = sizeof(int) == sizeof(long long) ? 0x3f3f3f3f3f3f3f3fLL : 0x3f3f3f3f;
const int MOD = (int)(1e9 + 7);

signed main() {
	vector<bool>v(1300000, true);
	v[0] = v[1] = false;
	for (int i = 2; i <= sqrt(1300000); i++) {
		if (v[i]) {
			for (int j = 2; i*j < 1300000; j++) {
				v[i*j] = false;
			}
		}
	}

	int x;
	while (cin >> x, x) {
		int ans = 0;
		for (int i = 0; v[x + i] == false; i++)ans++;
		for (int j = 0; v[x + j] == false; j--)ans++;
		cout << ans  << endl;

	}
	return 0;
}