// SGU 193 -- Chinese Girls' Amusement
import java.io.IOException;
import java.io.InputStream;
import java.io.InputStreamReader;
import java.io.BufferedReader;
import java.io.PrintWriter;
import java.util.StringTokenizer;
import java.math.BigInteger;
class Solver {
final static BigInteger TWO = BigInteger.valueOf(2);
void run(InputReader in, PrintWriter out) throws IOException {
BigInteger n = new BigInteger(in.nextToken());
BigInteger r = n.divide(TWO);
if (n.remainder(TWO).equals(BigInteger.ONE)) {
out.println(r);
} else {
r = r.subtract(BigInteger.ONE);
if (r.remainder(TWO).equals(BigInteger.ZERO)) {
r = r.subtract(BigInteger.ONE);
}
out.println(r);
}
out.flush();
}
}
public class Solution {
public static void main(String args[]) {
try {
new Solver().run(new InputReader(System.in), new PrintWriter(System.out));
} catch (IOException e) {
}
}
}
class InputReader {
BufferedReader reader;
StringTokenizer tokenizer;
InputReader(InputStream in) {
reader = new BufferedReader(new InputStreamReader(in));
tokenizer = new StringTokenizer("");
}
String nextToken() throws IOException {
while (!tokenizer.hasMoreTokens()) {
tokenizer = new StringTokenizer(reader.readLine());
}
return tokenizer.nextToken();
}
int nextInt() throws IOException {
return Integer.parseInt(nextToken());
}
}