#include <iostream>
#include <vector>
#include <map>
#include <bits/stdc++.h>
//#include <atcoder/dsu>
using namespace std;
//using namespace atcoder;
#define reps(i,s,n) for(int i = s; i < n; i++)
#define rep(i,n) for (int i = 0; i < (n); ++i)
#define ALL(v) v.begin(), v.end()
template<class T> inline bool chmin(T& a, T b) { if (a > b) { a = b; return 1; } return 0; }
using ll = long long;

ll A[200006];
double dp[200006][2];


int main() {
    ll N, M, K;
    cin >> N >> M >> K;
    
    rep(i, K){
        ll aa;
        cin >> aa;
        A[aa] = 1;
    }
    int r = 0;
    rep(i, N){
        if (A[i]){
            r++;
            if (r >= M){
                cout << -1;
                return 0;
            }
        } else {
            r = 0;
        }
    }
    double sum0 = 0.0, sum1 = 0.0;
    for (int i = N - 1; i >= 0; i--){
        if (A[i]){
            dp[i][0] = 0;
            dp[i][1] = 1;
        } else {
            dp[i][0] = sum0/M + 1.0;
            dp[i][1] = sum1/M;
        }
        //fprintf(stdout, "%d, %.3f, %.3f\n", i, dp[i][0], dp[i][1]);
        sum0 += dp[i][0] - dp[i + M][0];
        sum1 += dp[i][1] - dp[i + M][1];
    }
    double a = dp[0][0], b = dp[0][1];
    fprintf(stdout, "%.15f\n", a/(1.0 - b));
    return 0;
}

//@formatter:off
#include<bits/stdc++.h>
#define overload4(_1,_2,_3,_4,name,...) name
#define rep1(i,n) for (ll i = 0; i < ll(n); ++i)
#define rep2(i,s,n) for (ll i = ll(s); i < ll(n); ++i)
#define rep3(i,s,n,d) for(ll i = ll(s); i < ll(n); i+=d)
#define rep(...) overload4(__VA_ARGS__,rep3,rep2,rep1)(__VA_ARGS__)
#define rrep(i,n) for (ll i = ll(n)-1; i >= 0; i--)
#define all(a) a.begin(),a.end()
#define rall(a) a.rbegin(),a.rend()
#define popcount(x) __builtin_popcount(x)
#define pb push_back
#define eb emplace_back
#ifdef __LOCAL
#define debug(...) { cout << #__VA_ARGS__; cout << ": "; print(__VA_ARGS__); cout << flush; }
#else
#define debug(...) void(0)
#endif
#define INT(...) int __VA_ARGS__;scan(__VA_ARGS__)
#define LL(...) ll __VA_ARGS__;scan(__VA_ARGS__)
#define STR(...) string __VA_ARGS__;scan(__VA_ARGS__)
#define CHR(...) char __VA_ARGS__;scan(__VA_ARGS__)
#define DBL(...) double __VA_ARGS__;scan(__VA_ARGS__)
#define LD(...) ld __VA_ARGS__;scan(__VA_ARGS__)
using namespace std;
using ll = long long;
using ld = long double;
using P = pair<int,int>;
using LP = pair<ll,ll>;
using vi = vector<int>;
using vvi = vector<vector<int>>;
using vl = vector<ll>;
using vvl = vector<vector<ll>>;
using vd = vector<double>;
using vvd = vector<vector<double>>;
using vs = vector<string>;
using vc = vector<char>;
using vvc = vector<vector<char>>;
using vb = vector<bool>;
using vvb = vector<vector<bool>>;
using vp = vector<P>;
using vvp = vector<vector<P>>;
template<class S,class T> istream& operator>>(istream &is,pair<S,T> &p) { return is >> p.first >> p.second; }
template<class S,class T> ostream& operator<<(ostream &os,const pair<S,T> &p) { return os<<'{'<<p.first<<","<<p.second<<'}'; }
template<class T> istream& operator>>(istream &is,vector<T> &v) { for(T &t:v){is>>t;} return is; }
template<class T> ostream& operator<<(ostream &os,const vector<T> &v) { os<<'[';rep(i,v.size())os<<v[i]<<(i==int(v.size()-1)?"":","); return os<<']'; }
template<class T> bool chmin(T& a,T b) {if(a > b){a = b; return true;} return false;}
template<class T> bool chmax(T& a,T b) {if(a < b){a = b; return true;} return false;}
void scan(){}
template <class Head, class... Tail> void scan(Head& head, Tail&... tail){ cin >> head; scan(tail...); }
template<class T> void print(const T& t){ cout << t << '\n'; }
template <class Head, class... Tail> void print(const Head& head, const Tail&... tail){ cout<<head<<' '; print(tail...); }
template<class... T> void fin(const T&... a) { print(a...); exit(0); }
const string yes[] = {"no","yes"};
const string Yes[] = {"No","Yes"};
const string YES[] = {"NO","YES"};
const int inf = 1001001001;
const ll linf = 1001001001001001001;
//@formatter:on

int dp[1 << 17][17];

int main() {
    ios::sync_with_stdio(false);
    cin.tie(nullptr);
    cout.tie(nullptr);
    INT(n, m);
    vvi G(n);
    rep(i, m) {
        INT(a, b);
        a--, b--;
        G[a].pb(b);
        G[b].pb(a);
    }
    INT(k);
    vi c(k);
    cin >> c;
    rep(i, k) c[i]--;
    vvi dist(k, vi(n, inf));
    rep(i, k) {
        dist[i][c[i]] = 0;
        queue<int> q;
        q.push(c[i]);
        while (q.size()) {
            int u = q.front();
            q.pop();
            for (int v : G[u]) {
                if (chmin(dist[i][v], dist[i][u] + 1)) q.push(v);
            }
        }
    }
    rep(i, 1 << k) rep(j, k) dp[i][j] = inf;
    rep(i, k) dp[1 << i][i] = 0;
    rep(bit, 1 << k) rep(i, k) {
            int now = dp[bit][i];
            if (now == inf) continue;
            rep(j, k) {
                if (bit >> j & 1) continue;
                if (dist[i][c[j]] == inf) continue;
                chmin(dp[bit | 1 << j][j], now + dist[i][c[j]]);
            }
        }
    int ans = inf;
    rep(i, k) chmin(ans, dp[(1 << k) - 1][i]);
    print(ans == inf ? -1 : ans + 1);
}
