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>
#include <cstdio>
#include <iomanip>
#include <vector>
#include <map>
#include <set>
#include <queue>
#include <bitset>
#include <stack>
#include <utility>
#include <numeric>
#include <algorithm>
#include <functional>
#include <cctype>
#include <complex>
#include <string>
#include <sstream>

using namespace std;

#define all(c) c.begin(),c.end()
#define rall(c) c.rbegin(),c.rend()
#define mp(a,b) make_pair((a),(b))
#define eq ==

typedef long long ll;
typedef complex<double> point;
typedef pair<int,int> pii;

// →↑←↓
const int dx[] = {1,0,-1,0};
const int dy[] = {0,-1,0,1};


const double EPS = 1e-9;

const int M = 1299800;
bool isPrime[M];

vector<int> primes;

void sieve(){
    for(int i=2;i<M;i++) isPrime[i] = true;
    for(int i=2;i*i < M;i++){
        if(!isPrime[i]) continue;
        for(int j=i*i;j<M;j+=i){
            isPrime[j] = false;
        }
    }
    for(int i=2;i<M;i++) {
        if(isPrime[i]) primes.push_back(i);
    }
}

int main(){
    sieve();
    while(true){
        int n;
        cin >> n;
        if(n == 0) break;
        if(isPrime[n]){
            cout << 0 << endl;
        }else{
            cout << (*upper_bound(all(primes),n)) -  *(upper_bound(all(primes),n)-1) << endl;
        }
    }
    return 0;
}

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

using namespace std;

#define pb push_back
#define mp make_pair
#define fi first
#define se second
#define FOR(i, a, b) for(int i=(a);i<(b);i++)
#define REP(i, n) FOR(i, 0, n)
#define RFOR(i, a, b) for(int i=(a);i>=(b);i--)
#define RREP(i, n) RFOR(i, n, 0)
#define MFOR(i, m) for(auto i=(m).begin();i!=(m).end();i++)
#define ALL(a) (a).begin(), (a).end()
#define SZ(x) ((int)(x).size())

typedef long long int ll;
typedef pair<int, int> P;
typedef pair<ll, ll> Pll;
typedef vector<int> vi;
typedef vector<vi> vvi;
typedef vector<ll> vll;
typedef vector<vll> vvll;

const double eps = 1e-10;
const int MOD = 1000000007;
const int INF = 1000000000;
const ll LINF = 1 << 30;

template<typename T>
void printv(vector<T> const& s) {
  REP(i, SZ(s)) {
    cout << s[i] << " ";
  }
  cout << endl;
}

struct Sieve {
  vector<bool> isprime;

  void init(size_t max){
    if(max+1 > isprime.size()){     // resizeで要素数が減らないように
        isprime.resize(max+1,true); // IsPrimeに必要な要素数を確保
    }
    isprime[0] = false; // 0は素数ではない
    isprime[1] = false; // 1は素数ではない

    for(size_t i=2; i*i<=max; ++i) // 0からsqrt(max)まで調べる
        if(isprime[i]) // iが素数ならば
            for(size_t j=2; i*j<=max; ++j) // (max以下の)iの倍数は
                isprime[i*j] = false;      // 素数ではない
  }
};


int main () {
  cin.tie(0);
  cout << setprecision(10);

  Sieve s;
  s.init(1300000);

  vi p(100000), su(100000);
  p[0] = 2;
  su[0] = 0;
  int j = 1;
  FOR(i, 3, 1300000) {
    if(s.isprime[i]) {
      j++;
      p[j] = i;
      su[j] = i - p[j-1];
    }
  }

  while(1) {
    int n; cin >> n;
    if(n == 0) break;
    if(s.isprime[n]) {
      cout << 0 << endl;
      continue;
    }
    REP(i, 100000) {
      if(p[i] > n) {
        cout << su[i] << endl;
        break;
      }
    }
  }
  
}

C++: #include <iostream>
#include <vector>
#include <algorithm>

using namespace std;

int main(){
	const int N = 1299709 + 1;
	vector<int> primes;
	vector<bool> sieve(N, false);
	sieve[0] = sieve[1] = true;
	for(int i = 2; i < N; ++i){
		if(sieve[i]){ continue; }
		primes.push_back(i);
		for(int j = i + i; j < N; j += i){ sieve[j] = true; }
	}
	while(true){
		int k;
		cin >> k;
		if(k == 0){ break; }
		if(k < 2 || !sieve[k]){
			cout << 0 << endl;
		}else{
			vector<int>::iterator it =
				lower_bound(primes.begin(), primes.end(), k);
			int upper = *it;
			int lower = *--it;
			cout << upper - lower << endl;
		}
	}
	return 0;
}