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- import java.io.PrintWriter;
- import java.util.Arrays;
- import java.util.Scanner;
- public class H {
- static Scanner cin = new Scanner(System.in);
- static PrintWriter cout = new PrintWriter(System.out);
- static boolean[] prime = new boolean[1000000 + 5];
- static {
- Arrays.fill(prime, true);
- prime[0] = prime[1] = false;
- for (int i = 2; i * i <= prime.length; i++)
- if (prime[i])
- for (int j = i * i; j < prime.length; j += i) prime[j] = false;
- }
- public static void main(String[] args) {
- while (cin.hasNext()) {
- int n = cin.nextInt();
- int tot = 0;
- for (int i = 3; i <= n / 2; i += 2) {
- if (prime[i] && prime[n - i])
- ++tot;
- }
- cout.println(tot);
- }
- cin.close();
- cout.close();
- }
- }
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