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++: #define _CRT_SECURE_NO_WARNINGS // #pragma warning(disable:4996)
#include <iostream>
#include <cstdio>
#include <cstring>
#include <algorithm>
#include <vector>
#include <string>
#include <queue>
#include <functional>
#include <sstream>
#include <cmath>
#include <set>
#include <map>
#include <stack>
using namespace std; 

#define rep(i,a,b) for(int i=(a);i<(b);i++)
#define rrep(i,a,b) for(int i=(a);i>=(b);i--)
#define pb push_back
#define mp(a,b) make_pair(a,b)
#define all(a) a.begin(),a.end()
#define len(x) ((int)(x).size())
#define tmax(a,b,c) max((a),max((b),(c)))
#define tmin(a,b,c) min((a),min((b),(c)))
#define debug(x) cerr << #x << " is " << x << endl;

typedef pair<int, int> Pii;
typedef map<int, int> Mii;
typedef vector<int> Vi;
typedef long long ll;
const int inf = 2e9;
const ll ll_inf = 1e17;
const int mod = 1e9 + 7;
const long double eps = 1e-10;

int p[1322501];

int main()
{
	rep(i,2,1150) {
		int j = i;
		while (j < 1322500) {
			j += i;
			p[j]++;
		}
	}

	while (1) {
		int n; cin>>n;
		if (!n) break;
		if (!p[n]) cout << 0 << endl;
		else {
			int l=n, r=n;
			while (p[l]) l--;
			while (p[r]) r++;
			cout << r-l << endl;
		}
	}
	return 0;
}

C++: #define  _CRT_SECURE_NO_WARNINGS
#include "bits/stdc++.h"
#pragma comment (linker, "/STACK:526000000")

using namespace std;
typedef string::const_iterator State;
#define eps 1e-11L
#define MAX_MOD 1000000007LL
#define GYAKU 500000004LL

#define MOD 998244353LL
#define seg_size 262144*2LL
#define pb push_back
#define mp make_pair
typedef long long ll;
#define REP(a,b) for(long long (a) = 0;(a) < (b);++(a))
#define ALL(x) (x).begin(),(x).end()

unsigned long xor128() {
	static unsigned long x = 123456789, y = 362436069, z = 521288629, w = time(NULL);
	unsigned long t = (x ^ (x << 11));
	x = y; y = z; z = w;
	return (w = (w ^ (w >> 19)) ^ (t ^ (t >> 8)));
}

void init() {
	iostream::sync_with_stdio(false);
	cout << fixed << setprecision(10);
}

#define int ll
void solve() {
	while (true) {
		int k;
		cin >> k;
		if (k == 0) return;
		int left = k;
		int right = k;
		while (left != 1) {
			int ok = 0;
			for (int q = 2; q * q <= left; ++q) {
				if (left % q == 0) {
					ok = 1;
				}
			}
			if (ok == 0) break;
			left--;
		}
		while (true) {
			int ok = 0;
			for (int q = 2; q * q <= right; ++q) {
				if (right % q == 0) {
					ok = 1;
				}
			}
			if (ok == 0) break;
			right++;
		}
		cout << max(0LL,right - left) << endl;
	}
}
#undef int

int main() {
	init();
	solve();
}

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

int main() {
    array<bool, 2000000> is_prime;
    vector<int> prime;
    fill(is_prime.begin(), is_prime.end(), true);
    is_prime[0] = is_prime[1] = false;
    for(int i=2; i<is_prime.size(); ++i) {
        if(is_prime[i]) {
            prime.push_back(i);
            for(int j=i+i; j<is_prime.size(); j+=i) {
                is_prime[j] = false;
            }
        }
    }

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

C++: #include<bits/stdc++.h>
using namespace std;

typedef long long int ll;
typedef pair<int, int> pii;
typedef vector<int> vi;
typedef vector<pair<int, int> > vii;
#define rrep(i, m, n) for(int (i)=(m); (i)<(n);  (i)++)
#define erep(i, m, n) for(int (i)=(m); (i)<=(n); (i)++)
#define  rep(i, n)    for(int (i)=0; (i)<(n);  (i)++)
#define rrev(i, m, n) for(int (i)=(n)-1; (i)>=(m); (i)--)
#define erev(i, m, n) for(int (i)=(n); (i)>=(m); (i)--)
#define  rev(i, n)    for(int (i)=(n)-1; (i)>=0; (i)--)
#define vrep(i, c)    for(__typeof((c).begin())i=(c).begin(); i!=(c).end(); i++)
#define  ALL(v)       (v).begin(), (v).end()
#define pb            push_back
template<class T, class S> inline pair<T, S> mp(T x, S y){ return make_pair(x, y); }
template<class T, class S> inline bool minup(T& m, S x){ return m>(T)x ? (m=(T)x, true) : false; }
template<class T, class S> inline bool maxup(T& m, S x){ return m<(T)x ? (m=(T)x, true) : false; }

static const int    INF = 1000000000;
static const ll     MOD = 1000000007LL;
static const double EPS = 1E-12;

const int MAX_N = 1300000;
bool flag[MAX_N+1];
int prime[MAX_N+1];
int N;

int getPrime(int MAX_N = MAX_N, bool* flag = flag, int* prime = prime)
{
  int ptr = 0;
  prime[ptr++] = 2;
  flag[1] = true;
  for(int i=4; i<=MAX_N; i+=2) flag[i] = true;
  for(int i=3; i<=MAX_N; i+=2){
    if(!flag[i]){
      prime[ptr++] = i;
      for(int j=2*i; j<=MAX_N; j+=i) flag[j] = true;
    }
  }
  return ptr;
}


int main()
{
  int ptr = getPrime();

  while(cin >> N, N){
    if(flag[N]) cout << prime[upper_bound(prime, prime + ptr, N) - prime] - prime[lower_bound(prime, prime + ptr, N) - prime - 1] << endl;
    else cout << 0 << endl;
  }

  return 0;
}