#include <bits/stdc++.h>
using namespace std;
int main() {
  int N, K, A[8], times = 0;
  cin >> N >> K;
  for (int i = 0; i < N; i++) {
    cin >> A[i];
  }
  times++;
  N -= 3;
  while (N > 0) {
    times++;
    N -= 2;
  }
  cout << times;
  return 0;
}
