#include<bits/stdc++.h>
using namespace std;
struct job{
int time;
int idx;
bool operator < (const job & other){
return time < other.time;
}
};
job a[1000010];
int n, m, d;
bool check(int k){
int curtime = 1;
for(int i = 1; i <= m; i+=k){
if(a[i].time < curtime){
return false;
}
curtime++;
}
return true;
}
int main(){
ios::sync_with_stdio(false);
cin.tie(nullptr);
cin >> n >> d >> m;
for(int i = 1; i <= m; i++){
cin >> a[i].time;
a[i].time += d;
a[i].idx = i;
}
sort(a+1, a+m+1);
int l = 1, r = n;
int mid;
int ans = 0;
while(l <= r){
mid = (l+r)/2;
if(check(mid) == true){
ans = mid;
r = mid-1;
}else{
l = mid+1;
}
}
cout << ans;
return 0;
}
