#include <bits/stdc++.h>
using namespace std;
#define fi first
#define se second
#define pb push_back
typedef long long ll;
typedef pair<int, int> ii;
typedef vector<int> vi;
int N, D, M; // days, max delay, reqs
int numbers[1000050];
bool check(int n) { // checks if n machines is enough
int needed[N] = {0};
for (int i = 0; i < M; i++) { // finds earliest possible time for each
int day = numbers[i];
while (needed[day] == n)
day++;
if (day - numbers[i] > D)
return false;
needed[day]++;
}
return true;
}
int main() {
cin>>N>>D>>M;
for (int i = 0; i < M; i++) {
cin>>numbers[i];
}
sort(numbers, numbers + M);
int low = 0;
int high = 1e9;
int ans = -1;
while (low <= high) { // false, false, false, true, true, true
int mid = (low + high) / 2;
if (check(mid)) {
ans = mid;
high = mid - 1;
}
else {
low = mid + 1;
}
}
cout<<ans<<endl;
}
