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>

using namespace std;

bool sosuu[10000000] = {0};

int main() {
	fill(&sosuu[0], &sosuu[0]+10000000, true);
	sosuu[0] = false;
	sosuu[1] = false;
	for (int i = 2; i < 10000000; ++i) {
		for (int j = i*2; j < 10000000; j += i) {
			sosuu[j] = false;
		}
	}

	int n;
	while ( cin >> n, n ) {
		int l = n, r = n;
		for (; l > 0; --l) {
			if (sosuu[l]) {
				break;
			}
		}
		for (; r < 10000000; ++r) {
			if (sosuu[r]) {
				break;
			}
		}
		cout << r-l << endl;
	}
}

C++: #include <bits/stdc++.h>
using namespace std;
#define FOR(i, s, n) for(int i = s; i < (int)n; i++)
#define per(i, n) for(int i = n; i >= 0; i--)
#define ROF(i, s, n) for(int i = s; i >= (int)n; i--)
#define FORIT(i, A) for (auto i : A)
#define PRINT(x) cout << (x) << "\n"
#define ALL(a) (a).begin(),(a).end()
#define RALL(a) (a).rbegin(), (a).rend()
#define MP make_pair
#define EACH(i, n) for (__typeof((n).begin()) i = (n).begin(); i != (n).end(); ++i)
#define SZ(a) int((a).size())
#define EXIST(s,e) ((s).find(e)!=(s).end())
#define SORT(c) sort((c).begin(),(c).end())
#define CLR(a) memset((a), 0 ,sizeof(a))
#define NCLR(a) memset((a), -1 ,sizeof(a))
#define dump(x) cout << #x << " = " << (x) << "\n";
#define debug(x) cout << #x << " = " << (x) << " (L" << __LINE__ << ")" << " " << __FILE__ << "\n";
#define sq(n) (n) * (n)
typedef vector<int> VI;
typedef vector<VI> VVI;
typedef vector<string> VS;
typedef pair<int, int> PII;
typedef long long LL;
typedef vector<LL> VLL;
typedef vector<VLL> VVLL;
typedef unsigned int uint;
typedef unsigned long long ull;
typedef priority_queue<int> maxpq;
typedef priority_queue< int, vector<int>, greater<int> > minpq;
typedef complex<double> P;
static const double EPS = 1e-10;
static const double PI = acos( -1.0 );
static const int mod = 1000000007;
static const int INF = 1 << 29;
static const LL LL_INF = 1152921504606846976;
static const int dx[] = { -1, 0, 1, 0, 1, -1, 1, -1 };
static const int dy[] = { 0, -1, 0, 1, 1, 1, -1, -1 };



int main() {
	vector<bool> is_pm( 1299710, true );
	is_pm[ 0 ] = is_pm[ 1 ] = false;
	for ( int i = 2; i * i <= 1299709; i += 2 ) {
		if ( is_pm[ i ] ) {
			for ( int j = i * i; j <= 1299709; j += i ) {
				is_pm[ j ] = false;
				if ( i != 2 ) {
					j += i;
				}
			}
			if ( i == 2 ) {
				--i;
			}
		}
	}
	int n;
	while ( ~scanf( "%d", &n ) && n ) {
		int a, b;
		for ( int i = n; i >= 2; i-- ) {
			if ( is_pm[ i ] ) {
				a = i;
				break;
			}
		}
		for ( int i = n; i <= 1299709; i++ ) {
			if ( is_pm[ i ] ) {
				b = i;
				break;
			}
		}
		printf( "%d\n", b - a );
	}
	return 0;
}

C++: #include <bits/stdc++.h>

typedef long long LL;

//container util
//------------------------------------------
#define ALL(a)  (a).begin(),(a).end()
#define RALL(a) (a).rbegin(), (a).rend()
#define PB push_back
#define MP make_pair
#define DECIM8  fixed<<setprecision(8) 
#define SZ(a) int((a).size())
#define SORT(c) sort((c).begin(),(c).end())

//repetition
//------------------------------------------
#define FOR(i,a,b) for(int i=(a);i<(b);++i)
#define REP(i,n)  FOR(i,0,n)

//constant
//--------------------------------------------
const double EPS = 1e-10;
const double PI  = acos(-1.0);

//clear memory
#define CLR(a) memset((a), 0 ,sizeof(a))

using namespace std;
bool prime[1400000];
vector<int> primev;
int main(void)
{
  REP(i,1400000) prime[i]=true;
  prime[0]=false;
  prime[1]=false;
  REP(i,sqrt(1400000)+1) if(prime[i]) for(int j=i*i;j<1400000;j+=i) prime[j]=false;
  REP(i,1400000) if(prime[i]) primev.PB(i);
  for(;;){
    int n;
    cin >> n;
    if(!n) return 0;
    if(n<=2) cout << 0 << endl;
    else cout << *(lower_bound(primev.begin(),primev.end(),n))-*(upper_bound(primev.begin(),primev.end(),n)-1) << endl;
  }
}

C++: #include <iostream>
#include <string>
#include <vector>
#include <algorithm>
#include <cmath>
#include <cstdio>
#include <functional>
#include <numeric>
#include <stack>
#include <queue>
#include <map>
#include <set>
#include <utility>
#include <sstream>
#include <complex>
#include <fstream>
#include <bitset>
#include <time.h>

using namespace std;

typedef long long ll;
typedef pair<ll, ll> P;
typedef vector<ll> V;
typedef complex<double> Point;

#define PI acos(-1.0)
#define EPS 1e-10
const ll INF = (1LL << 31) - 1;
const ll MOD = 1e9 + 7;

#define FOR(i,a,b) for(int i=(a);i<(b);i++)
#define rep(i,N) for(int i=0;i<(N);i++)
#define ALL(s) (s).begin(),(s).end()
#define EQ(a,b) (abs((a)-(b))<EPS)
#define EQV(a,b) ( EQ((a).real(), (b).real()) && EQ((a).imag(), (b).imag()) )
#define fi first
#define se second
#define N_SIZE (1LL << 20)
#define NIL -1
#define MAX_N 100100 * 3

vector<int> prime;    
bool is_prime[2000000];//is_prime[i]==1??????i????´???°

int sieve(int n) {//n??\???????´???°?????°?????????
	int p = 0;
	for (int i = 0; i <= n; i++)is_prime[i] = 1;
	is_prime[0] = is_prime[1] = 0;
	for (int i = 2; i <= n; i++) {
		if (is_prime[i]) {
			prime.push_back(i);
			for (int j = 2 * i; j <= n; j += i)is_prime[j] = 0;
		}
	}
	return p;
}

int n;

int main() {
	sieve(1300000);
	while (cin >> n&&n) {
		if (is_prime[n]) {
			cout << 0 << endl;
			continue;
		}
		rep(i, prime.size() - 1) {
			if (prime[i] < n&&n < prime[i + 1]) {
				cout << prime[i + 1] - prime[i] << endl;
				break;
			}
		}
	}
}