#include <bits/stdc++.h>
using namespace std;
int a[380];
int main() {
  int n, k, m, ans = 0, i;
  scanf("%d%d%d", &n, &k, &m);
  for (i = 1; i <= m; i++) {
    scanf("%d", &a[i]);
    ans = ans + 1 + (a[i] - a[i - 1] - 1) / k;
  }
  ans = ans + (n - a[m]) / k;
  printf("%d", ans);
  return 0;
}
