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.util.*;

public class Main {
	
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		
		int primax = 1500000;
		int[] pri = new int[primax+10000];
		for(int i=2;i<=primax;i++) pri[i] = i;
		for(int i=2;i<=primax+1;i++){
			for(int j=i+i;j<=primax;j+=i){
					pri[j]=0;
			}
		}

		while(true){
			int n = sc.nextInt();
			if(n==0)break;
			if(pri[n]>0){
				System.out.println("0");
			}else{
				int a = 0;
				int b = 0;
				for(int i=n;;i++){
					if(pri[i]>0){
						a = i;
						break;
					}
				}
				for(int i=n;;i--){
					if(pri[i]>0){
						b = i;
						break;
					}
				}
				System.out.println(a-b);
			}
		}
	}
	
}