#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][3];
ll dist[N][3];
struct Node {
    int u;
    ll dis;
    int fa;
    int layer;  // 在哪一层
    Node(int u, ld dis, int fa, int la): u(u), dis(dis), fa(fa), layer(la) {}
};
struct cmp {
    bool operator()(Node& n1, Node& n2) {
        return n1.layer == n2.layer ? n1.dis > n2.dis: n1.layer < n2.layer;
    }
};
void Dijkstra(int s, int n) {
    fori(1, n) {
        vis[i][0] = vis[i][1] = vis[i][2] = false;
        dist[i][0] = dist[i][1] = dist[i][2] = 1e18;
    }
    priority_queue<Node,vector<Node>,cmp> pri;
    pri.push(Node(s, 0, 0, 0));
    while (pri.size()) {
        auto no = pri.top();
        pri.pop();
        if (vis[no.u][no.layer]) continue;
        vis[no.u][no.layer] = true;
        dist[no.u][no.layer] = no.dis;
        // debug(no.u);
        // debug(no.dis);
        // debug(no.layer);

        if (no.layer == 1) {  // 刚刚转移下来
            for (auto [v, b]: g[no.u]) {
                if (vis[v][2]) continue;
                ll dis2 = (h[no.fa]-h[v])*(h[no.fa]-h[v]);
                pri.push(Node(v, no.dis+dis2, no.u, 2));
            }
            continue;
        }
        // 同一层
        for (auto [v, dis]: g[no.u]) {
            if (vis[v][no.layer]) continue;
            pri.push(Node(v, no.dis+dis, no.u, no.layer));
        }
        if (no.layer == 0) {  // 跳到下一层
            for (auto [v, b]: g[no.u]) {
                if (vis[v][1]) continue;
                pri.push(Node(v, no.dis, no.u, 1));
            }
        }
    }
}
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});
    }
    Dijkstra(s, n);
    cout << fixed << setprecision(10);
    cout << min({dist[t][0]/2.0l, dist[t][1]/2.0l, dist[t][2]/2.0l})+1e-10 << endl;
    // cout << dist[t][2]/2.0l << endl;
    
    return 0;
}