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.Arrays;
import java.util.Scanner;

public class Main {

	static boolean[] isPrime=new boolean[12997090];
	static{
		Arrays.fill(isPrime,true);
		isPrime[0]=isPrime[1]=false;
		for(int i = 2;i*i<=isPrime.length;i++){
			if(isPrime[i]){
				for(int j = i+i;j<isPrime.length;j+=i){
					isPrime[j]=false;
				}
			}
		}
		
	}
	public static void main(String[] args) {
		Scanner cin = new Scanner(System.in);
		while(true){
			int N =cin.nextInt();
			if(N==0){
				break;
			}
			int min,max;
			min=max=N;
			for(;max<isPrime.length;max++){
				if(isPrime[max]){
					break;
				}
			}
			for(;min>0;min--){
				if(isPrime[min]){
					break;
				}
			}
			System.out.println(max-min);
		}

	}

}