#include <cstdio>

#include <cstring>

#include <cmath>

#include <algorithm>

#include <queue>

#include <cstdlib>

#include <vector>

#include <set>

#include <map>

#define LL long long

#define inf 1<<30

using namespace std;

int n,m,l,a[];

int judge(int x)

{

    int res=,temp=;

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

    {

        if(a[i]-temp>x)return ;

        while(a[i]-temp<=x&&i<=n)i++;

        temp=a[i-];i--;

        res++;

    }

    return res<=m;

}

int main()

{

    while(scanf("%d%d%d",&l,&n,&m)>)

    {

        for(int i=;i<=n;i++)scanf("%d",&a[i]);

        a[++n]=l;

        sort(a+,a+n+);

        int left=,right=l,mid,ans;

        while(left<=right)

        {

            mid=(left+right)/;

            if(judge(mid))

                right=mid-,ans=mid;

            else left=mid+;

        }

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

    }

}