#include <bits/stdc++.h>
using namespace std;
#define pb push_back
#define mp make_pair
typedef long long ll;
typedef pair<long long, long long> pll;
typedef pair<int,int> pii;
typedef vector<long long> vl;
typedef vector<int> vi;
int n, d, m;
vector<pii> arr;
bool check(int machines){
int job = m-1;
for(int day = n; day >= 1; day--){
int cnt = 0;
while(arr[job].first - day <= d && cnt + 1 <= machines){
job--;
cnt++;
}
if(job < 0) return true;
}
return false;
}
int main(){
cin>>n>>d>>m;
arr.resize(m);
for(int i = 0; i < m; i++)
{
cin>>arr[i].first;
arr[i].second = i;
}
sort(arr.begin(), arr.end());
int l = 1, r = n, ans = -1;
while(l <= r){
int mid = (l + r) / 2;
bool can = check(mid);
if(can){
ans = mid;
r = mid - 1;
}
else
l = mid + 1;
}
cout<<ans<<endl;
return 0;
}
