#include <bits/stdc++.h>

using namespace std;

const int MAXN = 50010;
int L,N,M;
int D[MAXN];

bool check(int d)
{
  int cnt = 0;
  int last = 0;
  for(int i = 1;i<=N+1;i++)
  {
    int current = (i == N+1) ? L : D[i];
    if(current - last<d)
    {
      cnt ++;
    }
    else {
      last = current;
    }
    if(cnt>M)
    return false;
  }
  return true;
}

int main()
{
  cin>>L>>N>>M;
  for(int i = 1;i<=N;i++)
  {
    cin>>D[i];
  }
  int left = 0,right = L;
  int ans = 0;
  while(right-left>=0)
  {
    int mid = (right-left)/2+left;
    if(check(mid))
    {
      ans = mid;
      left = mid+1;
    }
    else 
    {
      right = mid-1;
    }
  }
  cout << ans << endl;
  return 0;
}