#include <bits/stdc++.h>
using namespace std;
int n, m, c;
bool pos[505];
int main() {
  int i, j, k, t;
  scanf("%d%d", &n, &m);
  scanf("%d", &c);
  for (i = 0, t = 0; i < c; i++) {
    scanf("%d", &k);
    pos[k] = 1;
  }
  for (i = 1, k = t = 0; i <= n; i++) {
    if (pos[i]) {
      t++;
      k = 0;
    } else {
      k++;
      if (k >= m) {
        k = 0;
        t++;
      }
    }
  }
  printf("%d\n", t);
  return 0;
}
