#include <bits/stdc++.h>
using namespace std;

int L, n, m;
int d[50001];
int ans = 0;

bool check(int dist){
	int now = 0;
	int cnt = 0;
	for(int i=1; i<=n; i++){
		if(d[i]-d[now] < dist){
			cnt++;
		}
		else now = i;
	}
	if(cnt <= m) return true;
	return false;
}

int main(){
	cin>>L>>n>>m;
	for(int i=1; i<=n; i++){
		cin>>d[i];
	}
	d[0] = 0;
	d[n+1] = L;
	if(n == 0){
		cout<<L;
		return 0;
	}
	int l=1, r=L;
	while(l < r){
		int mid = (l+r+1)/2;
		if(check(mid)) l = mid;
		else r = mid-1;
	}
	ans = l;
	cout<<ans;
	return 0;
}