#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
const int N = 500010;
int a[N];
int k,n,m;

bool check(int x)
{
   int now=0,nex=0,tot=0;
   while(now<n+1)
   {
        nex++;
        if(a[nex]-a[now]<x)
            tot++;
        else
            now=nex;
   }
    return tot<=m;
}

int main()
{
    cin>>k>>n>>m;
    for(int i=1;i<=n;i++) cin>>a[i];
    a[n+1]=k;
    int l=1,r=k;
    
    while(l<r)
    {
        int mid = l+r+1>>1;
        if(check(mid))
            l = mid;
        else
            r = mid - 1;
    }
        
    cout<<l<<endl;
    return 0;
}