#include "bits/stdc++.h"
using namespace std;
#define ff first
#define ss second
#define int long long
#define pii pair<int,int>
#define pb emplace_back
#define rep(i,a,b) for(int i=a; i < b; ++i)
#define owo ios::sync_with_stdio(0); cin.tie(0); cout.tie(0);

#define cerr if(1) cout
#define dbg(x) cerr << "?" << #x << " : " << x << endl;

void solve();
signed main(){
	owo
	int t = 1; //cin >> t
	while(t--) solve();
}

double dist[100005][2];
vector<int> adj[100005];

void solve(){
	int n, m, s, t; cin >> n >> m >> s >> t;
	vector<int> H(n + 1);
	for(int i = 1; i <= n; ++i){
		cin >> H[i];
		dist[i][0] = dist[i][1] = 1e18;
	}
	dist[s][0] = dist[s][1] = 0;
	for(int i = 1; i <= m; ++i){
		int u, v; cin >> u >> v;
		adj[u].pb(v);
		adj[v].pb(u);
	}
	priority_queue<pair<double ,pii>, vector<pair<double, pii>>, greater<pair<double, pii>>> pq;
	pq.push(make_pair(0, pii(s, 0)));
	while(pq.size()){
		auto [d, P] = pq.top();
		pq.pop();
		auto [u, p] = P;
		if(u < 0 && d > dist[-u][1]) continue;
		if(u < 0 && d <= dist[-u][1]){
			for(auto v:adj[-u]){
				double w = H[-u] - H[v];
				w *= w;
				if(w + d < dist[v][1]){
					dist[v][1] = w + d;
					pq.push(make_pair(dist[v][1], pii(-v, u)));
				}
			}
			continue;
		}
		double tw = (H[u] - H[p]);
		tw *= tw;
		if(d <= dist[u][0]){
			for(auto v:adj[u]){
				double w = H[u] - H[v];
				w *= w;
				if(w + d < dist[v][0]){
					dist[v][0] = w + d;
					dist[v][1] = min(dist[v][1], w + d);
					pq.push(make_pair(dist[v][0], pii(v, u)));
				}
			}
		}
		if(d - tw <= dist[p][0]){
			for(auto v:adj[u]){
				double w = (H[p] - H[v]);
				w *= w;
				w /= 2.0;
				double w2 = H[u] - H[v];
				w2 *= w2;
				double nd = min(dist[u][0], min(w, w2) + dist[p][0]);
				if(nd < dist[v][1]){	
					dist[v][1] = nd;	
					pq.push(make_pair(dist[v][1], pii(-v, u)));
				}
			}
		}
	}
	cout << fixed << setprecision(6) << dist[t][1] << '\n';
}