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 <vector>
#include <algorithm>
#include <numeric>
#include <set>
#include <map>
#include <string>
#include <stack>
#include <queue>
#include <cmath>
#include <cstdio>
#include <istream>
#include <sstream>
#include <iomanip>
#include <iterator>
#include <climits>
using namespace std;

typedef ostringstream OSS;
typedef istringstream ISS;

typedef vector<int> VI;
typedef vector< VI > VVI;

typedef long long LL;

typedef pair<int, int> PII;
typedef vector<PII> VPII;

#define X first
#define Y second

vector<bool> Prime;
vector<int> PrimeStk;
void initPrime(long num) {
	Prime = vector<bool>(num + 1, true);
	Prime[1] = Prime[0] = false;
	long sq_num = sqrt((double)num);
	for (long i = 2; i < sq_num; i++) {
		if (Prime[i]) {
			for (long j = i + i; j <= num; j += i) {
				Prime[j] = false;
			}
		}
	}
}
void initPrimeStk(long begin) {
	PrimeStk.clear();
	for (long i = begin; i < (int)Prime.size(); i++) {
		if (Prime[i]) {
			PrimeStk.push_back(i);
		}
	}
}

int main(void) {
	initPrime(2000000);
	initPrimeStk(0);

	int N;
	while (cin >> N, N) {
		auto it = lower_bound(PrimeStk.begin(), PrimeStk.end(), N);

		if (*it == N) {
			cout << 0 << endl;
			continue;
		}

		cout << ((*it) - *(it - 1)) << endl;
	}

	return 0;
}

C++: #include<iostream>
#include<string>
using namespace std;
int main(){
	int n;
	while(true){
		cin>>n;
		if(n==0)
			break;
		int minprime=n;
		while(true){
			bool t=true;
			for(int i=2;i*i<=minprime;i++){
				if(minprime%i==0)
					t=false;
			}
			if(t==true)
				break;
			minprime--;
		}
		int maxprime=n;
		while(true){
			bool t=true;
			for(int i=2;i*i<=maxprime;i++){
				if(maxprime%i==0)
					t=false;
			}
			if(t==true)
				break;
			maxprime++;
		}
		cout<<maxprime-minprime<<endl;
	}
	return 0;
}