#include <bits/stdc++.h>
using namespace std;
long long int m, n, c, d, j, k, l, r, x, t, y, u, a[5000], b, z, i, e, f;
int main() {
  cin >> n >> k >> c;
  for (i = 1; i <= c; i++) {
    cin >> a[i];
    d += (a[i] - a[i - 1] - 1) / k + 1;
  }
  cout << d + (n - a[c]) / k;
}
