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

10112724921700

Sample Output

4060114

Problem Source

Tokyo 2007

// source code of submission 966492, Zhongshan University Online Judge System#include <iostream>#include <cmath>using namespace std;int prime[100001];bool ispri(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(ispri(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;}