#include <bits/stdc++.h>
using namespace std;
#define fori(x, y) for (int i=(x);i<=(y);++i)
#define forj(x, y) for (int j=(x);j<=(y);++j)
#define fork(x, y) for (int k=(x);k<=(y);++k)

typedef long long ll;
typedef pair<int,int> pii;
typedef pair<ll,ll> pll;

#define debug(x) cerr << #x << " = " << x << endl

typedef long double ld;
const int N = 2e5+7;
ll h[N];
vector<pll> g[N];

bool vis[N];
ll dist[N][2];
struct Node {
    int u;
    ll dis;
    int fa;
    // int layer;  // 在哪一层
    Node(int u, ld dis, int fa): u(u), dis(dis), fa(fa) {}
};
struct cmp {
    bool operator()(Node& n1, Node& n2) {
        return n1.dis > n2.dis;
    }
};
void dij(int s, int n, int lay) {
    fori(1, n) {
        vis[i] = false;
        dist[i][lay] = 1e18;
    }
    priority_queue<Node,vector<Node>,cmp> pri;
    pri.push(Node(s, 0, 0));
    while (pri.size()) {
        auto no = pri.top();
        pri.pop();
        int u = no.u;
        if (vis[u]) continue;
        dist[u][lay] = no.dis;
        vis[u] = true;
        for (auto [v, w]: g[u]) {
            if (vis[v]) continue;
            pri.push(Node(v, no.dis+w, u));
        }
    }
}
int main() {
    ios::sync_with_stdio(0);
    cin.tie(0);
    cout.tie(0);

    int n, m, s, t;
    cin >> n >> m >> s >> t;
    fori(1, n) cin >> h[i];
    fori(1, m) {
        int u, v;
        cin >> u >> v;
        ll dis = (h[u]-h[v])*(h[u]-h[v])*2;
        g[u].push_back({v, dis});
        g[v].push_back({u, dis});
    }
    dij(s, n, 0);
    dij(t, n, 1);
    ll ans = min(dist[t][0], dist[s][1]);
    for (auto [v, w]: g[s]) {
        if (v == t) {
            ans = 0;
            break;
        }
    }
    fori(1, n) {
        if (i == s||i == t) continue;
        if (g[i].size()>500) {
            vector<bool> vis(n+1, false);
            vector<ll> dist1(n+1, 1e18);
            priority_queue<Node,vector<Node>,cmp> pri;
            pri.push(Node(s, 0, 0));
            while (pri.size()) {
                auto no = pri.top();
                pri.pop();
                int u = no.u;
                if (vis[u]) continue;
                dist1[u] = no.dis;
                vis[u] = true;
                for (auto [v, w]: g[u]) {
                    if (vis[v]) continue;
                    if (u == i) {
                        pri.push(Node(v, no.dis+(h[u]-h[no.fa])*(h[u]-h[no.fa]), u));
                    }
                    else if (v == i) pri.push(Node(v, no.dis, u));
                    else pri.push(Node(v, no.dis+w, u));
                }
            }
            ans = min(ans, dist1[t]);
        }
        else {
            forj(0, g[i].size()-1) fork(j+1, g[i].size()-1) {
                int u = g[i][j].first, v = g[i][k].first;
                if (dist[u][0]+dist[v][1]>dist[v][0]+dist[u][1]) swap(u, v);
                ans = min(ans, dist[u][0]+dist[v][1]+(h[u]-h[v])*(h[u]-h[v]));
            }
        }
    }
    cout << fixed << setprecision(10);
    cout << ans/2.0l << endl;

    return 0;
}