Problem:
Problem B: Prime Gap

The sequence of n - 1 consecutive composite numbers (positive integers that are not prime and
not equal to 1) lying between two successive prime numbers p and p + n is called a prime gap of length n. For example, (24, 25, 26, 27, 28) between 23 and 29 is a prime gap of length 6.

Your mission is to write a program to calculate, for a given positive integer k, the length of the prime gap that contains k. For convenience, the length is considered 0 in case no prime gap
contains k.

Input

The input is a sequence of lines each of which contains a single positive integer. Each positive
integer is greater than 1 and less than or equal to the 100000th prime number, which is 1299709.
The end of the input is indicated by a line containing a single zero.

Output

The output should be composed of lines each of which contains a single non-negative integer. It
is the length of the prime gap that contains the corresponding positive integer in the input if it
is a composite number, or 0 otherwise. No other characters should occur in the output.

Sample Input

10
11
27
2
492170
0

Output for the Sample Input

4
0
6
0
114

Solution in JAVA (judge verdict: Accepted)

import java.io.*;
import java.math.*;
import java.util.*;
import java.util.regex.Matcher;
import java.util.regex.Pattern;
import java.lang.ArrayIndexOutOfBoundsException;

/**
 * @author yoshikyoto
 */
class Main extends MyUtil{
	// public static ArrayList<ArrayList<Integer>> g;
	public static Graph g;
	public static int[][] arr = new int[11175][3];
	public static ArrayComp cmp = new ArrayComp();
	
	public static void main(String[] args) throws Exception{
		Prime p = new Prime(1300000);
		while(true){
			int n = readInt();
			if(n == 0) break;
			if(p.a[n]){
				System.out.println(0);
				continue;
			}
			int cnt = 0;
			for(int i = n; !p.a[i]; i--){
				cnt++;
			}
			for(int i = n+1; !p.a[i]; i++){
				cnt++;
			}
			System.out.println(cnt+1);
		}
	}
	
	public static int bs(int sq, int h, int l, int r){
		// System.out.println("bs " + sq + " " + h + " " + l + " " + r);
		if(l == r) return l;
		int c = (l + r) / 2;
		boolean left_flag = false;
		if(arr[c][0] < sq){
			left_flag = true;
		}else if(arr[c][0] == sq && arr[c][1] <= h){
			left_flag = true;
		}
		if(left_flag){
			return bs(sq, h, l, c);
		}else{
			return bs(sq, h, c+1, r);
		}
	}
}

class Data extends MyUtil{
	String name;
	int time, finishedAt;
	Data(String str){
		String[] strs = str.split(" ");
		name = strs[0];
		time = parseInt(strs[1]);
	}
}


class DataComparator implements Comparator<Data>{
    public int compare(Data a, Data b) {
    	return a.finishedAt - b.finishedAt;
    	// a-bで昇順
    	/*
        if (a.point != b.point) {
            return a.point - b.point;
        } else {
        	return a.name.compareTo(b.name);
        }
        */
    }
}

class ArrayComp implements Comparator<int[]>{
	public int compare(int[] a, int[] b){
		int l = Math.min(a.length, b.length);
		for(int i = 0; i < l; i++){
			if(a[i] == b[i]) continue;
			return b[i] - a[i];
		}
		return 0;
	}
}

class Node extends ArrayList<Edge>{
	int index, depth = -1, dist = -1;
	Node(int index){this.index = index;}
	Node parent;
	boolean visited = false;
}
class Edge{
	int from, to, cost;
	Edge(int from, int to, int cost){
		this.from = from;
		this.to = to;
		this.cost = cost;
	}
}
class NodeComparator implements Comparator<Node>{
	public int compare(Node a, Node b){
		return a.dist - b.dist;
	}
}

class Graph{
	Node n[];
	Graph(int node_count){
		n = new Node[node_count];
		for(int i = 0; i < node_count; i++) n[i] = new Node(i);
	}
	public void add(Edge e){
		n[e.from].add(e);
	}
	public Node get(int i){return n[i];}
	public Node lca(int a, int b){
		// 浅い方をaとする
		Node nodeA, nodeB;
		if(n[a].depth < n[b].depth){
			nodeA = n[a];
			nodeB = n[b];
		}else{
			nodeA = n[b];
			nodeB = n[a];
		}
		// 同じ深さまで親をたどる
		int diff = nodeB.depth - nodeA.depth;
		for(int k = 0; k < diff; k++){
			nodeB = nodeB.parent;
		}
		// 共通祖先を見つける
		while(nodeA != nodeB){
			nodeA = nodeA.parent;
			nodeB = nodeB.parent;
		}
		return nodeA;
	}
	public void calcDepth(int root){
		ArrayDeque<Integer> que = new ArrayDeque<Integer>();
		que.push(root);
		n[root].depth = 0;
		
		while(que.size() > 0){
			int curr = que.pop();
			Node curr_node = n[curr];
			for(Edge e : curr_node){
				int next = e.to;
				Node next_node = n[next];
				if(next_node.depth == -1){
					next_node.depth = curr_node.depth + 1;
					next_node.parent = curr_node;
					que.push(next);
				}
			}
		}
	}
	
	public int[] dijkstra(int s){
		PriorityQueue<Node> q = new PriorityQueue<Node>(n.length, new NodeComparator());
		Node start_node = new Node(s);
		start_node.dist = 0;
		q.add(start_node);
		int[] dist = new int[n.length];
		for (int i = 0; i < dist.length; i++)  dist[i] = -1;
		dist[s] = 0;
		
		while(q.size() > 0){
			Node currNode = q.poll();
			if(dist[currNode.index] < currNode.dist) continue;
			for(Edge e : n[currNode.index]){
				Node nextNode = new Node(e.to);
				nextNode.dist = currNode.dist + e.cost;
				if(dist[e.to] == -1 || dist[e.to] > nextNode.dist){
					dist[e.to] = nextNode.dist;
					q.add(nextNode);
				}
			}
		}
		return dist;
	}
}



class Regex{
	Pattern p; Matcher m; String str;
	Regex(String regex_str){p = Pattern.compile(regex_str);}
	void setStr(String str){m = p.matcher(str);}
	boolean find(){return m.find();}
	String group(int i){return m.group(i);}
	String group(){return m.group();}
}

/**
 * UnionFindTree 
 * @author yoshikyoto
 */
class UnionFindTree{
    public int[] parent, rank;
    public int n;
    public int count;
    // 初期化
    UnionFindTree(int n){
        this.n = n;
        count = n;
    	parent = new int[n];
    	rank = new int[n];
        for(int i = 0; i < n; i++){
            parent[i] = i;
            rank[i] = 0;
        }
    }
    // 根を求める
    int find(int x){
        if(parent[x] == x){
            return x;
        }else{
            return parent[x] = find(parent[x]);
        }
    }
    // xとyの集合を結合
    void unite(int x, int y){
        x = find(x);
        y = find(y);
        if(x == y){
            return;
        }
        if(rank[x] < rank[y]){
            parent[x] = y;
            count--;
        }else{
            parent[y] = x;
            if(rank[x] == rank[y]) rank[x]++;
            count--;
        }
    }
    // xとyが同じ集合か
    boolean same(int x, int y){
        return find(x) == find(y);
    }
};


/**
 * 整数を数え上げたりするクラス
 * new Prime(int n) でnまでエラトステネスの篩を実行。
 * @author yoshikyoto
 * @param a[i] iが素数の時true
 * @param count[i] i以下の素数の数
 */
class Prime{
	boolean[] a;
	int[] count;
	Prime(int n){
		a = new boolean[n+1];
		a[0] = false; a[1] = false;
		for(int i = 2; i <= n; i++) a[i] = true;
		// ふるい
		for(int i = 2; i < (n - 3) / 2; i++)
			if(a[i]) for(int j = 2; j * i <= n; j++)
					a[j * i] = false;
		
		// 数え上げ
		count = new int[n+1];
		count[0] = 0;
		for(int i = 1; i <= n; i++){
			int gain = 0;
			if(a[i]) gain = 1;
			count[i] = count[i-1] + gain;
		}
	}
	boolean isPrime(int i){return a[i];}
	static boolean calcPrime(int i){
		if(i <= 1) return false;
		if(i == 2) return true;
		if(i % 2 == 0) return false;
		for(int j = 3; j <= Math.sqrt(i); j+=2){
			if(i % j == 0) return false;
		}
		return true;
	}
}

class CountHashMap<E> extends HashMap<E, Integer>{
	ArrayList<E> keyArray = new ArrayList<E>();
	public void add(E key){add(key, 1);}
	public void add(E key, Integer value){
		if(containsKey(key)){value += get(key);
		}else{keyArray.add(key);}
		put(key, value);
	}
}

/**
 * MyUtil
 * @author yoshikyoto
 */
class MyUtil extends MyIO{
	public static Random rand = new Random();
	public static int digit(int n){return String.valueOf(n).length();}
	public static String reverse(String s){return (new StringBuffer(s)).reverse().toString();}
	public static void dsort(int[] a){
		Arrays.sort(a);
		int l = a.length;
		for(int i = 0; i < l/2; i++){
			int tmp = a[i]; a[i] = a[l-1-i]; a[l-1-i] = tmp;
		}
	}
	public static void sleep(int t){try{Thread.sleep(t);}catch(Exception e){}}
	public static int sum(int[] a){int s = 0; for(int i = 0; i < a.length; i++)s+=a[i]; return s;}
	public static int[] cp(int[] a){
		int[] b = new int[a.length];
		for(int i = 0; i < a.length; i++) b[i] = a[i];
		return b;
	}
	public static int randomInt(int min, int max){return min + rand.nextInt(max - min + 1);}
	static boolean inside(int y, int x, int h, int w){return 0 <= y && y <= (h-1) && 0 <= x && x <= (w-1);};
	public static boolean isUpper(char c){return 'A'<=c&&c<='Z';}
	public static boolean isLower(char c){return 'a'<=c&&c<='z';}
	public static char toUpper(char c){
		if(isLower(c)) return (char)(c - 'a' + 'A');
		return c;
	}
	public static char toLower(char c){
		if(isUpper(c)) return (char)(c - 'A' + 'a');
		return c;
	}
	public static int[] swap(int[] arr, int i, int j){
		int[] ret = cp(arr);
		int tmp = ret[i];
		ret[i] = ret[j];
		ret[j] = tmp;
		return ret;
	}
}

/**
 * MyIO
 * @author yoshikyoto
 */
class MyIO extends MyMath{
	static Scanner sc = new Scanner(new InputStreamReader(System.in));
	public static char scNextChar(){return sc.next().charAt(0);}
	static BufferedReader br = new BufferedReader(new InputStreamReader(System.in));
	public static String line;
	
	public static int ins[];
	public static int[] readIntMap() throws IOException{return parseInt(readLine().split(" "));}
	public static int readIntMap(int i) throws Exception{
		if(i == 0) ins = readIntMap();
		return ins[i];
	}
	public static int[] readIntToMap(int n) throws IOException{
		int[] ret = new int[n];
		for(int i = 0; i < n; i++) ret[i] = readInt();
		return ret;
	}
	public static int[] readNoDistIntMap() throws IOException{
		String[] strs = readLine().split("");
		int l = strs.length;
		int[] ret = new int[l-1];
		for(int i = 1; i < l; i++)
			ret[i-1] = parseInt(strs[i]);
		return ret;
	}
	public static boolean readToLine() throws IOException{return (line = br.readLine()) != null;}
	public static String readLine() throws IOException{return br.readLine();}
	public static int readInt() throws IOException{return Integer.parseInt(br.readLine());}
	public static int[] parseInt(String[] arr){
		int[] res = new int[arr.length];
		for(int i = 0; i < arr.length; i++)res[i] = Integer.parseInt(arr[i]);
		return res;
	}
	public static double[] parseDouble(String[] arr){
		double[] res = new double[arr.length];
		for(int i = 0; i < arr.length; i++)res[i] = Double.parseDouble(arr[i]);
		return res;
	}
	public static boolean[] parseBool(String[] arr){
		int[] t = parseInt(arr);
		boolean[] res = new boolean[t.length];
		for(int i = 0; i < t.length; i++){
			if(t[i] == 1){res[i] = true;
			}else{res[i] = false;}
		}
		return res;
	}
	public static int parseInt(Object o){return Integer.parseInt(o.toString());}
}

class MyMath{
	public static int max(int[] arr){return max(arr, 0, arr.length-1);}
	public static int max(int[] arr, int l, int r){
		int max = arr[l];
		for(int i = l+1; i <= r; i++)
			max = Math.max(max, arr[i]);
		return max;
	}
	public static int min(int[] arr){return min(arr, 0, arr.length-1);}
	public static int min(int[] arr, int l, int r){
		int min = arr[l];
		for(int i = l+1; i <= r; i++)
			min = Math.min(min, arr[i]);
		return min;
	}
	public static boolean isCross(int x1, int y1, int x2, int y2, int x3, int y3, int x4, int y4){
	    // 並行な場合
	    int m = (x2-x1)*(y4-y3)-(y2-y1)*(x4-x3);
	    if(m == 0) return false;
	    // 媒介変数の値が0より大きく1以下なら交差する、これは問題の状況によって変わるかも。
	    double r =(double)((y4-y3)*(x3-x1)-(x4-x3)*(y3-y1))/m;
	    double s =(double)((y2-y1)*(x3-x1)-(x2-x1)*(y3-y1))/m;
	    return (0 < r && r <= 1 && 0 < s && s <= 1);
	}
	public static boolean isParallel(int x1, int y1, int x2, int y2, int x3, int y3, int x4, int y4){
	    if((x2-x1)*(y4-y3) == (y2-y1)*(x4-x3)) return true;
	    else return false;
	}
	public static double sq(double d){return d*d;}
	public static int sq(int i){return i*i;}
	public static int sqdist(int x1, int y1, int x2, int y2){return (x1-x2) + sq(y1-y2);}
	public static double sqdist(double x1, double y1, double x2, double y2){return sq(x1-x2) + sq(y1-y2);}
	public static double dist(int x1, int y1, int x2, int y2){return Math.sqrt(sqdist(x1, y1, x2, y2));}
	public static double dist(double x1, double y1, double x2, double y2){return Math.sqrt(sqdist(x1, y1, x2, y2));}
	long comb(long n, long m){
		if(n < m) return 0;
    	long c = 1; m = (n - m < m ? n - m : m);
    	for(long ns = n - m + 1, ms = 1; ms <= m; ns ++, ms ++){c *= ns; c /= ms;}
    	return c;
    }
	static int gcd(int a, int b){if(a%b==0){return(b);}else{return(gcd(b,a%b));}};
	/**
	 * aの中から、合計がborderを超えるペア（重複除く）の数を数える
	 */
	static int countPair(int[] a, int border){
		int count = 0, l = a.length, i = 0, j = l-1;
		for (; i < l; i++) {
			for (; j >= 0; j--) if(a[i] + a[j] <= border) break;
			count += l - (j + 1);
			if(j < i) count--;
		}
		return count/2;
	}
}