import java.util.Scanner;
 
public class Main {
	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);
		int n, count = 0;
 
		while (true) {
			n = sc.nextInt();
			int[] arr = new int [n*2+1];
			for (int i = 2; i <= (n * 2); i++) {
				arr[i] = i;
			}
 
			for (int i = 2; i <= (n * 2); i++) {
				if (arr[i] == 0) continue;
				for (int j = i + i; j <= (n * 2); j+=i) {
					arr[j] = 0;
				}
			}
			for (int i = n+1; i <= (n * 2); i++) {
				if (arr[i] != 0) {
					count++;
				}
			}
			System.out.println(count);
			count = 0;
		}
	}
}

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