import java.util.*;
import java.lang.*;
import java.io.*;
public class jobs {
static int n,m,d;
static int [] jobs;
public static void main(String[] args) throws IOException{
Scanner sc = new Scanner(System.in);
n =sc.nextInt();
d =sc.nextInt();
m =sc.nextInt();
jobs =new int [m];
for(int i =0;i<m;i++) {
jobs[i]=sc.nextInt();
}
Arrays.sort(jobs);
int l=0;
int h=jobs[n-1];
while(l<h) {
int mid = (l+h)/2;
if(works(mid)) {
h=mid;
}
else {
l=mid+1;
}
}
System.out.println(l);
}
static boolean works(int mid) {
int currday=jobs[0];
int machs=0;
int i =0;
while(i<m) {
if(currday>=jobs[i]-1) {
if(currday-jobs[i]>d) {
return false;
}
if(machs<mid) {
machs++;
i++;
}
else {
machs=0;
currday++;
}
}
else {
currday=jobs[i];
}
}
return true;
}
}
