#include <bits/stdc++.h>
int main() {
  int n, k, c;
  scanf("%d%d%d", &n, &k, &c);
  int i, x = 0;
  int gifts = 0;
  for (i = 0; i < c; i++) {
    int y, z = 0;
    scanf("%d", &y);
    while (x != y) {
      x++;
      z++;
      if ((z == k) && (x != y)) {
        gifts++;
        z = 0;
      }
    }
    gifts++;
  }
  gifts += (n - x) / k;
  printf("%d", gifts);
  return 0;
}
