//Слава Україні, Героям слава

#include <bits/stdc++.h>

using namespace std;

const int N = 1e5 + 7;

struct line {
    long long b, m;
};

int nh, pos;
line hull[N], v[N];
pair<long long, int> tmp[N];

bool check(line s, line t, line u) {
    return (s.b - t.b) * (u.m - s.m) < (s.b - u.b) * (t.m - s.m);
}

void update(line s) {
    while(nh >= 2 and !check(hull[nh - 2], hull[nh - 1], s)) {
        nh--;
    }
    hull[nh++] = s;
}

long long eval(int id, long long x) {
    return hull[id].b + hull[id].m * x;
}

long long query(long long x) {
    while(pos + 1 < nh and eval(pos, x) < eval(pos + 1, x)) {
        pos++;
    }
    return -eval(pos, x);
}

int main() {
    ios_base::sync_with_stdio(false);
    cin.tie(nullptr);
    int n, m, s, t;
    cin >> n >> m >> s >> t;
    vector<long long> h(n + 1);
    vector<vector<int>> adj(n + 1);
    for(int i = 1; i <= n; i++) {
        cin >> h[i];
        h[i] <<= 1;
    }
    bool foi = false;
    for(int i = 0; i < m; i++) {
        int a, b;
        cin >> a >> b;
        adj[a].push_back(b);
        adj[b].push_back(a);
        if((a == s and b == t) or (a == t and b == s)) {
            foi = true;
        }
    }
    if(foi) {
        cout << 0 << "\n";
        return 0;
    }
    vector<vector<long long>> dist(2, vector<long long>(n + 1, LLONG_MAX));
    priority_queue<pair<long long, int>, vector<pair<long long, int>>, greater<pair<long long, int>>> pq;
    dist[0][s] = dist[1][t] = 0;
    pq.push({0, s});
    while(!pq.empty()) {
        auto p = pq.top(); pq.pop();
        for(auto e : adj[p.second]) {
            if(dist[0][e] > p.first + (h[e] - h[p.second]) * (h[e] - h[p.second])) {
                dist[0][e] = p.first + (h[e] - h[p.second]) * (h[e] - h[p.second]);
                pq.push({dist[0][e], e});
            }
        }
    }
    pq.push({0, t});
    while(!pq.empty()) {
        auto p = pq.top(); pq.pop();
        for(auto e : adj[p.second]) {
            if(dist[1][e] > p.first + (h[e] - h[p.second]) * (h[e] - h[p.second])) {
                dist[1][e] = p.first + (h[e] - h[p.second]) * (h[e] - h[p.second]);
                pq.push({dist[1][e], e});
            }
        }
    }
    long long ans = LLONG_MAX;
    int sz = 0;
    for(int i = 1; i <= n; i++) {
        if(adj[i].size() == 1 or i == s or i == t) {
            continue;
        }
        nh = pos = sz = 0;
        for(auto e : adj[i]) {
            v[sz] = {-h[e] * h[e] / 2 - dist[0][e], h[e]};
            tmp[sz++] = {h[e], e};
        }
        sort(v, v + sz, [](line s, line t) {
            return (s.m == t.m) ? (s.b < t.b) : (s.m < t.m);
        });
        sort(tmp, tmp + sz);
        for(int i = 0; i < sz; i++) {
            update(v[i]);
        }
        for(int i = 0; i < sz; i++) {
            long long cur = query(tmp[i].first) + dist[1][tmp[i].second] + tmp[i].first * tmp[i].first / 2;
            ans = min(ans, cur);
        }
    }
    cout << ans / 4 << (ans % 4 == 2 ? ".5\n" : "\n");
    return 0;
}