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 long long ll;
typedef vector<int> VI;
typedef vector<VI> VVI;
typedef vector<ll> VL;
typedef vector<VL> VVL;
typedef pair<int, int> PII;

#define FOR(i, a, n) for (ll i = (ll)a; i < (ll)n; ++i)
#define REP(i, n) FOR(i, 0, n)
#define ALL(x) x.begin(), x.end()
#define MP make_pair
#define PB push_back
#define MOD 1000000007
#define INF (1LL<<30)
#define LLINF (1LL<<60)
#define PI 3.14159265359
#define EPS 1e-12
//#define int ll

int prime[1300000];
signed main(void)
{
  prime[0] = prime[1] = 1;
  for (int i = 2; i * i <= 1300000; i++) {
  	if (!prime[i]) {
  		for (int j = 2 * i; j <= 1300000; j += i) {
  			prime[j] = 1;
  		}
  	}
  }

  //REP(i, 20) cout << prime[i] << " "; cout << endl;
  FOR(i, 1, 1300000) {
    if(prime[i] > 0) prime[i] = prime[i-1] + 1;
  }
  //REP(i, 20) cout << prime[i] << " "; cout << endl;
  for(int i=1299998; i>=0; --i) {
    if(prime[i+1] && prime[i]) prime[i] = prime[i+1];
  }
  //REP(i, 20) cout << prime[i] << " "; cout << endl;

  while(true) {
    int n;
    cin >> n;
    if(!n) break;
    if(prime[n]) {
      cout << prime[n] + 1 << endl;
    } else {
      cout << 0 << endl;
    }
  }

  return 0;
}

C++: #include <iostream> 
#include <vector>
using namespace std;
const int MAX = 1299710;
int prime[MAX];

int cal(int n){
  int l = 0;
  int f = 0;
  for(l = n;!prime[l];l++){
  }
  for(f = n;!prime[f];f--){
  }
  return l - f;
  
}
int main(){
  prime[1] = 0;
  prime[2] = 1;
  fill(prime,prime+MAX,1);
  for(int i = 2;i < MAX;i++){
    if(prime[i]){
      for(int j = i+i;j < MAX;j+=i){
	prime[j] = 0;
      }
    }
  }
  /*  for(int i = 1;i < 100;i++){
    if(prime[i])cout << i << endl;
    }*/
  int n;
  while(cin >> n,n != 0){
    cout << cal(n) << endl;
  }

  return 0;
}

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

//define
#define ALL(a) a.begin(),a.end()
#define REP(i,n) for(int i=0;i<n;i++)
#define RREP(i,n) for(int i=n-1;i>=0;i--)
#define debug(x) if(1) cout<<#x<<":"<<x<<endl;
#define DEBUG(x) if(1) cout<<#x<<":"<<x<<endl;
#define ll long long

//constant
const int MOD=1000000007;
const int INF=INT_MAX/3-1;
const double EPS=1e-9;
const int dx[]={1,0,-1,0};
const int dy[]={0,1,0,-1};

//typedef
typedef vector<int> vi;
typedef vector<vector<int>> vvi;


bool isnprime[20000000];

int main(){
    isnprime[0]=1;
    isnprime[1]=1;
    vi primes;
    for(int i=2;i<2000000;i++)
        if(!isnprime[i]){
            primes.push_back(i);
            for(int j=i*2;j<2000000;j+=i)
                isnprime[j]=1;
        }
    int k;
    while(cin>>k,k){
        auto it=lower_bound(ALL(primes),k);
        if(*it==k)
            cout<<0<<endl;
        else
            cout<<*it-*(it-1)<<endl;





    }
    return 0;
    }

C++: #include <cstdio>
#include <vector>
#include <algorithm>
#include <array>
#include <set>
#include <map>
#include <queue>
#include <tuple>
#include <unordered_set>
#include <unordered_map>
#include <functional>
#include <cassert>
#define repeat(i, n) for (int i = 0; (i) < int(n); ++(i))
#define repeat_from(i, m, n) for (int i = (m); (i) < int(n); ++(i))
#define repeat_reverse(i, n) for (int i = (n)-1; (i) >= 0; --(i))
#define repeat_from_reverse(i, m, n) for (int i = (n)-1; (i) >= int(m); --(i))
#define whole(f, x, ...) ([&](decltype((x)) whole) { return (f)(begin(whole), end(whole), ## __VA_ARGS__); })(x)
#define unittest_name_helper(counter) unittest_ ## counter
#define unittest_name(counter) unittest_name_helper(counter)
#define unittest __attribute__((constructor)) void unittest_name(__COUNTER__) ()
using ll = long long;
using namespace std;
template <class T> inline void setmax(T & a, T const & b) { a = max(a, b); }
template <class T> inline void setmin(T & a, T const & b) { a = min(a, b); }
template <typename X, typename T> auto vectors(X x, T a) { return vector<T>(x, a); }
template <typename X, typename Y, typename Z, typename... Zs> auto vectors(X x, Y y, Z z, Zs... zs) { auto cont = vectors(y, z, zs...); return vector<decltype(cont)>(x, cont); }

vector<bool> sieve_of_eratosthenes(int n) { // enumerate primes in [2,n] with O(n log log n)
    vector<bool> is_prime(n+1, true);
    is_prime[0] = is_prime[1] = false;
    for (int i = 2; i*i <= n; ++i)
        if (is_prime[i])
            for (int k = i+i; k <= n; k += i)
                is_prime[k] = false;
    return is_prime;
}
vector<int> list_primes(int n) {
    auto is_prime = sieve_of_eratosthenes(n);
    vector<int> primes;
    for (int i = 2; i <= n; ++i)
        if (is_prime[i])
            primes.push_back(i);
    return primes;
}

int main() {
    vector<int> primes = list_primes(1299709);
    while (true) {
        int n; scanf("%d", &n);
        if (n == 0) break;
        auto it = whole(lower_bound, primes, n);
        int q = *it;
        int p = *it == n ? *it : *(-- it);
        printf("%d\n", q - p);
    }
    return 0;
}