Sicily 1500. Prime Gap

Description

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
 Copy sample input to clipboard

10
11
27
2
492170
0

Sample Output

4
0
6
0
114

Problem Source: Tokyo 2007

// Problem#: 1500
// Submission#: 3685345
// The source code is licensed under Creative Commons Attribution-NonCommercial-ShareAlike 3.0 Unported License
// URI: http://creativecommons.org/licenses/by-nc-sa/3.0/ // All Copyright reserved by Informatic Lab of Sun Yat-sen University
#include<iostream>
#include<iomanip>
#include<string>
#include<cstdio>
#include<cctype>
#include<cstring>
#include<algorithm>
#include<cmath>
using namespace std;

int prime[100001];
bool isPrime(int n){
for(int i=2;i<=sqrt(double(n));i++)
if(n%i==0)
return false;
return true;
}
void getPrime(){
int l=0;
for(int i=2;i<=1299709;i++)
if(isPrime(i))
prime[l++]=i;
}
int main(){
getPrime();
int k;
while(cin>>k&&k){
int i=0;
for(;i<100000;i++)
if(prime[i]>=k)
break;
if(prime[i]==k)
cout<<0<<endl;
else
cout<<prime[i]-prime[i-1]<<endl;
}
return 0;
}