#include <bits/stdc++.h>
using namespace std;
#define int long long
int n, d, m;
pair <int, int> arr[1000001] = {};
int bs (int pos, int x) {
int l = 1, r = m;
int mid, ans = m + 1;
while (l <= r) {
mid = (l + r) >> 1;
if (arr[mid].first > x) {
r = mid - 1;
ans = mid;
} else {
l = mid + 1;
}
}
return ans;
}
bool check (int x) {
int cnt = 1;
int i = 1;
int a;
while ((a = min((int)i + x, (int)bs(i + 1, arr[i].first + d))) != m + 1) {
cnt++;
i = a;
}
return (cnt <= n);
}
signed main () {
cin >> n >> d >> m;
map <int, vector <int>> positions;
for (int i = 1; i <= m; i++) {
cin >> arr[i].first;
arr[i].second = i;
positions[arr[i].first].push_back(i);
}
sort(arr + 1, arr + m + 1);
int l = 1, r = n;
int mid, ans;
while (l <= r) {
mid = (l + r) >> 1;
if (check(mid)) {
r = mid - 1;
ans = mid;
} else l = mid + 1;
}
cout << ans << endl;
}
