#include <bits/stdc++.h>
using namespace std;
#define int long long
using ll = long long;
using pii = pair<ll,ll>;
struct L{
	ll m, k;
	L(){

	}
	L(ll n, ll k){
		this->m = n;
		this->k = k;
	}
	ll get(ll x){
		return 0ll + m * x + k;
	}
};
using piL = pair<ll,L>;
ll banana(L a, L b){
	return (b.k - a.k + (a.m - b.m - 1)) / (a.m - b.m) ; 
}
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];

	vector<vector<int>> v(n+1);
	for(int i = 0;i < m;i ++){
		int a,b;
		cin >> a >> b;
		v[a].push_back(b);
		v[b].push_back(a);
	}
	
	priority_queue<pii, vector<pii>, greater<pii>> pq;
	vector<bool> vis(n+1);
	vector<ll> d1(n+1, 1e11);
	vector<ll> d2(n+1, 1e11);
	d1[s] = 0;
	d2[t] = 0;
	pq.emplace(0, s);
	while(pq.size() > 0){
		auto cur = pq.top();
		pq.pop();
		if(vis[cur.second])
			continue;
		vis[cur.second] = 1;
		for(int i : v[cur.second]){
			if(vis[i])
				continue;
			ll nx = cur.first + (h[i] - h[cur.second]) * (h[i] - h[cur.second]);
			if(nx < d1[i]){
				d1[i] = nx;
				pq.emplace(nx, i);
			}
		}
	}
	vis.clear();
	vis.resize(n+1);
	pq.emplace(0, t);
	while(pq.size() > 0){
		auto cur = pq.top();
		pq.pop();
		if(vis[cur.second])
			continue;
		vis[cur.second] = 1;

		for(int i : v[cur.second]){
			if(vis[i])
				continue;

			ll nx = cur.first + (h[i] - h[cur.second]) * (h[i] - h[cur.second]);
			if(nx < d2[i]){
				d2[i] = nx;
				pq.emplace(nx, i);
			}
		}
	}

	ll ans = 2*d1[t];

	for(int i = 1;i <= n;i ++){
		if(i == s || i == t){
			ll rec = 1e11;

			for(int it : v[i]){
				if(i == s)
					rec = min(rec, d2[it]*2);
				else
					rec = min(rec, d1[it]*2);
			}
			ans = min(ans, rec);
			continue;
		}
		deque<pair<int, L>> dq;
		set<pii> st;
		for(int it : v[i])
			st.emplace(h[it], it);
		map<int,int> mp;
		ll rec = 1e11;
		for(pii i : st){
			if(mp[i.first])
				continue;
			mp[i.first] = 1;
			L nx(-2 * i.first, 2*d2[i.second] + i.first * i.first);
			if(dq.size() == 0){
				dq.emplace_back(1e9, nx);
				continue;
			}
			
			while(dq.size() > 1){
				auto cur = dq.back();
				ll tmp = banana(cur.second, nx);
				if(tmp <= dq[dq.size()-2].first){
					dq[dq.size()-2].first = banana(nx, dq[dq.size()-2].second);
					dq.pop_back();
				}else{
					break;
				}
			}
			ll x = banana(nx, dq.back().second); 
			dq.back().first = x;
			dq.emplace_back(1e9, nx);
		}
		for(int p : v[i]){
			int l = 0;
			int r = dq.size();
			while(r - l > 1){
				int mid = (r + l) / 2;
				if(dq[mid].first >= h[p]){
					r = mid;
				}else{
					l = mid;
				}
			}
			rec = min(rec,  2*d1[p] + h[p] * h[p] + dq[r].second.get(h[p]));
		}
		ans = min(rec, ans);
	}
	cout << double(ans) / 2 << "\n";
}
signed main(){
	cin.tie(0)->sync_with_stdio(0);
	cout<<fixed << setprecision(6) ;
	int t = 1;
//	cin >> t;
	while(t--){
		solve();
	}
}