#include<bits/stdc++.h>
using namespace std;
int n, d, m, day[1000100];
vector<int> times[100100];
int main(){
cin.tie(nullptr)->sync_with_stdio(false);
cin >> n >> d >> m;
for(int i = 1; i <= m; ++i){
cin >> day[i];
times[day[i]].push_back(i);
}
int l = 1, r = 1e9;
while(l < r){
int mid = (l + r) >> 1, check = true;
queue<int> q;
for(int i = 1; i<=n; ++i){
int k = mid;
for(auto x:times[i]) q.push(x);
while(k-- && !q.empty()) q.pop();
if(!q.empty() && day[q.front()] + d == i){
check = false;
break;
}
}
if(check) r = mid;
else l = mid+1;
}
cout << l;
}
