#include<bits/stdc++.h>
using namespace std;
int L,M,N,a[50005];
int main(){
	cin>>L>>N>>M;
	for(int i=1;i<=N;i++)
	    cin>>a[i];
	int left=0,right=L;
	while(left<=right){
		int mid=(left+right)/2;
		int k=0,ans=0;
		for(int i=1;i<=N;i++){
			if(a[i]-a[k]<mid)ans++;
			else k=i;
		}
		if(ans>M)right=mid-1;
		else left=mid+1;
	}
	cout<<left-1;
	return 0;
}