#include <bits/stdc++.h>
int main() {
  int n, k, c;
  std::cin >> n >> k >> c;
  int holidays[c];
  int tpresents = 0;
  for (int i = 0; i < c; i++) {
    std::cin >> holidays[i];
  }
  if (c == 0) {
    tpresents += n / k;
  } else {
    tpresents += holidays[0] / k;
    if (holidays[0] % k != 0) {
      tpresents++;
    }
    int i;
    for (i = 1; i < c; i++) {
      tpresents += (holidays[i] - holidays[i - 1]) / k;
      if ((holidays[i] - holidays[i - 1]) % k != 0) {
        tpresents++;
      }
    }
    tpresents += (n - holidays[i - 1]) / k;
  }
  std::cout << tpresents << std::endl;
  return 0;
}
