#include <bits/stdc++.h>

using namespace std;
using int64 = long long;

constexpr int DEBUG = 0;

int main() {
  ios::sync_with_stdio(false);
  cin.tie(0);

  cout << fixed << setprecision(10);

  int n, m, k;
  cin >> n >> m >> k;
  vector<int> is_back(n + 1);
  for (int i = 0; i < k; i++) {
    int p;
    cin >> p;
    is_back[p] = 1;
  }

  int c = 0;
  for (int i = 0; i < n; i++) {
    if (is_back[i]) {
      c++;
      if (c >= m) {
        cout << -1 << endl;
        return 0;
      }
    } else {
      c = 0;
    }
  }

  vector<double> xs(n + 1);
  vector<double> ys(n + 1);
  int l = n;
  for (int i = n - 1; i >= 0; i--) {
    if (is_back[i]) {
      xs[i] = 1.0;
      ys[i] = 0.0;
    } else {
      if (l == n) {
        for (int j = i + 1; j <= n - 1; j++) {
          xs[i] += xs[j] / m;
          ys[i] += ys[j] / m;
        }
        ys[i] += 1.0;
      } else {
        xs[i] = xs[l];
        ys[i] = ys[l];
        for (int j = i + 1; j <= l; j++) {
          xs[i] += xs[j] / m;
          ys[i] += ys[j] / m;
        }
        for (int j = i + m + 1; j <= l + m; j++) {
          xs[i] -= xs[min(j, n)] / m;
          ys[i] -= ys[min(j, n)] / m;
        }
      }

      l = i;
    }
  }

  double a = xs[0];
  double b = ys[0];
  cout << b / (1.0 - a) << endl;
}
#include <bits/stdc++.h>
using namespace std;

typedef long long ll;
#define REP(i,n) for(int i=0,_n=(int)(n);i<_n;++i)
#define ALL(v) (v).begin(),(v).end()
#define CLR(t,v) memset(t,(v),sizeof(t))
template<class T1,class T2>ostream& operator<<(ostream& os,const pair<T1,T2>&a){return os<<"("<<a.first<<","<<a.second<< ")";}
template<class T>void pv(T a,T b){for(T i=a;i!=b;++i)cout<<(*i)<<" ";cout<<endl;}
template<class T>void chmin(T&a,const T&b){if(a>b)a=b;}
template<class T>void chmax(T&a,const T&b){if(a<b)a=b;}

ll nextLong() { ll x; scanf("%lld", &x); return x;}

int solve(int r1, int c1, int r2, int c2) {
  if (r1 == r2 && c1 == c2) return 0;
  if (abs(r1 - r2) + abs(c1 - c2) <= 3) return 1;
  if ((r1 + c1) == (r2 + c2)) return 1;
  if ((r1 - c1) == (r2 - c2)) return 1;
  if (abs((r1 + c1) - (r2 + c2)) <= 6) return 2;
  if (abs((r1 - c1) - (r2 - c2)) <= 6) return 2;
  if ((abs(r1+c1) - abs(r2+c2)) % 2 == 0) return 2;
  if ((abs(r1-c1) - abs(r2-c2)) % 2 == 0) return 2;
  return 3;
}

int main2() {
  ll r1 = nextLong();
  ll c1 = nextLong();
  ll r2 = nextLong();
  ll c2 = nextLong();

  cout << solve(r1, c1, r2, c2) << endl;
  return 0;
}

int main() {

#ifdef LOCAL
  for (;!cin.eof();cin>>ws)
#endif
    main2();
  return 0;
}