#include <bits/stdc++.h>
using namespace std;
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 = -1;
                        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";
}
int main(){
        cin.tie(0)->sync_with_stdio(0);
        int t = 1;
//      cin >> t;
        while(t--){
                solve();
        }
}