#include<iostream>

#include<cstdio>

#include<algorithm>

#include<cmath>

#include<cstring>

#include<string>

#include<cstdlib>

#include<stack>

#include<vector>

#include<queue>

#include<deque>

#include<map>

#include<set>

using namespace std;

#define lck_max(a,b) ((a)>(b)?(a):(b))

#define lck_min(a,b) ((a)<(b)?(a):(b))

typedef long long LL;

const int maxn=5e5+;

LL d,n,m;

LL t[maxn];

inline LL read()

{

    LL kr=,xs=;

    char ls;

    ls=getchar();

    while(!isdigit(ls))

    {

        if(!(ls^))

            kr=-;

        ls=getchar();

    }

    while(isdigit(ls))

    {

        xs=(xs<<)+(xs<<)+(ls^);

        ls=getchar();

    }

    return xs*kr;

}

inline void out(LL xs)

{

    if(!xs) {putchar(); return;}

    if(xs<) putchar('-'),xs=-xs;

    int kr[],ls=;

    while(xs) kr[++ls]=xs%,xs/=;

    while(ls) putchar(kr[ls]+),ls--;

}

LL cnt,i,now;

inline bool check(LL u)

{

    cnt=,i=,now=;

    while(i<=n)

    {

        i++;

        if(t[i]-t[now]<u)

            cnt++;

        else

            now=i;

    }

    if(cnt>m)

        return false;

    else

        return true;

}

LL ans;

inline void query(LL l,LL r)

{

    while(l<=r)

    {

        LL mid=l+r>>;

        if(check(mid))

            ans=mid,l=mid+;

        else

            r=mid-;

    }

}

int main()

{

    d=read();n=read();m=read();

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

        t[i]=read();

    t[n+]=d;//终点到起点的距离。

    query(,d);

    out(ans),putchar('\n');

return ;

}