Sicily 1500. Prime Gap

Time Limit: 1 secs, Memory Limit: 32 MB

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

10
11
27
2
492170
0
Sample Output

4
0
6
0
114
Problem Source

Tokyo 2007

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(～￣▽￣)～ Just do it! 暴力就好

#include <iostream>using namespace std;bool isPrime(int num){    for (int i = 2; i * i <= num; i++)        if (num % i == 0)            return false;    return true;}int main(){    int num, start, end;    bool sky;    while (cin >> num && num)    {        if (isPrime(num))            cout << 0 << endl;        else        {            end = num;            start = num;            while (!isPrime(end))                end++;            while (!isPrime(start))                start--;            cout << end - start << endl;        }    }}