#include <bits/stdc++.h>
using namespace std;
#define int long long
int n, d, m;
int arr[1000001] = {};
bool check (int x) {
int cnt = 0;
for (int day = 1; day <= n; day++) {
int z = cnt;
for (int i = cnt + 1; i <= min(m, cnt + x); i++) {
if (day + d < arr[i]) break;
z++;
}
cnt = z;
if (cnt == m) return true;
}
return false;
}
signed main () {
cin >> n >> d >> m;
for (int i = 1; i <= m; i++) {
cin >> arr[i];
}
sort(arr + 1, arr + m + 1);
int l = 1, r = 10;
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;
}
