import java.util.*;

public class Euler123 {
    public static void main(String[] args) {
        long limit = 10000000000L;
        List<Integer> primes = new ArrayList<>();
        for (int c = 2;; c++) {
            boolean prime = true;
            for (int p : primes) {
                if ((long) p * p > c)
                    break;
                if (c % p == 0) {
                    prime = false;
                    break;
                }
            }
            if (!prime)
                continue;
            primes.add(c);
            int n = primes.size();
            if ((n & 1) == 0)
                continue;
            if (2L * n * c > limit) {
                System.out.println(n);
                return;
            }
        }
    }
}