#include <iostream>

#include <cstdio>

#include <algorithm>

using namespace std;

int x[];

int l, n, m;

bool solve(int d)

{

        int last = ;

        for (int i = ; i < n - m; i++)

        {

               int cur = last + ;

               while (cur < n&&x[cur] - x[last] < d)

               {

                       cur++;

               }

               if (cur == n) return false;

               last = cur;

        }

        return true;

}

int main()

{

        scanf("%d%d%d", &l, &n, &m);

        for (int i = ; i <= n; i++)

               scanf("%d", &x[i]);

        ++n;

        x[n] = l;

        ++n;

        sort(x, x + n);

        int lb = , ub = l + ;

        while (ub - lb > )

        {

               int mid = (ub + lb) / ;

               if (solve(mid)) lb = mid;

               else ub = mid;

        }

        printf("%d\n", lb);

        return ;

}