#include <bits/stdc++.h>
long a[505] = {0};
int main() {
  long n, m, i, j;
  long x, c;
  long s = 0;
  scanf("%ld%ld", &n, &m);
  scanf("%ld", &c);
  for (i = 1; i <= c; i++) {
    scanf("%ld", &x);
    a[x] = 1;
    s++;
  }
  for (i = 1; i + m - 1 <= n; i++) {
    for (j = i; j <= i + m - 1; j++)
      if (a[j]) break;
    if (j > i + m - 1) {
      a[i + m - 1] = 1;
      s++;
    }
  }
  printf("%ld\n", s);
  return 0;
}
