// #pragma GCC optimizer("Ofast")
// #pragma GCC target("avx2")
#include <bits/stdc++.h>
// #define int long long 
#define ld long double
#define ull unsigned long long 
#define fr first
#define fi first
#define sc second
#define se second
#define all(x) x.begin(), x.end()
#define pii pair<int,int>
//#define sz(x) (int)x.size()
using namespace std;

#ifndef fireball
#define tofu ios::sync_with_stdio(0); cin.tie(0);
#else
#define tofu freopen("input.txt", "r", stdin); freopen("output.txt", "w", stdout);
#endif 

void db() {cout << '\n';}
template <typename T, typename ...U> void db(T a, U ...b) {cout << a << ' ', db(b...);}

#define ll long long 

const ll INF = 1ll << 60;
const int N = 1e5 + 5;
struct line{
    ll m, k;
    ll val(ll x){
        return 1ll * m * x + k;
    }
};
struct seg{
    int l, r, m;
    line li = {0, INF};
    seg* ch[2]={NULL, NULL};
    void insert(line _li){
        if(l == r){
            if(li.val(m) > _li.val(m)) swap(li,_li);//
            return;
        }
        if(_li.val(m) < li.val(m)) swap(_li,li);
        if(li.m <= _li.m){
            if(!ch[0]) ch[0] = new seg(l,m);
            ch[0]->insert(_li);
        }
        else{
            if(!ch[1]) ch[1] = new seg(m+1,r);
            ch[1]->insert(_li);
        }
    }
    ll query(ll p){
        if(l == r) return li.val(p);
        if(p <= m and ch[0]) return min(li.val(p),ch[0]->query(p));
        else if(p > m and ch[1]) return min(li.val(p),ch[1]->query(p));
        else return li.val(p);
    }
    seg(int _l,int _r){
        l=_l,r=_r;
        m=l+r>>1;
    }
};

signed main(){
    tofu;
    int n, m, S, T;
    cin >> n >> m >> S >> T;

    ll ans = INF;

    int h[n + 1] = {};
    for(int i = 1; i <= n; ++i) cin >> h[i];
    vector<int> g[n + 1];
    for(int i = 0; i < m; ++i){
        int x, y;
        cin >> x >> y;
        g[x].push_back(y);
        g[y].push_back(x);
        if(x == S and y == T) ans = 0;
        if(x == T and y == S) ans = 0;
    }

    vector<ll> ds(n + 1, INF), dt(n + 1, INF);
    vector<int> vs(n + 1, 0), vt(n + 1, 0);
    priority_queue<pair<ll, int>, vector<pair<ll, int> >, greater<pair<ll, int> > > q;

    ds[S] = 0;
    q.push({0, S});
    while(!q.empty()){
        int u = q.top().se;
        q.pop();
        if(vs[u]) continue;
        vs[u] = 1;
        for(int i : g[u]){
            if(ds[i] > ds[u] + 1ll * (h[i] - h[u]) * (h[i] - h[u])){
                ds[i] = ds[u] + 1ll * (h[i] - h[u]) * (h[i] - h[u]);
                q.push({ds[i], i});
            }
        }
    }

    dt[T] = 0;
    q.push({0, T});
    while(!q.empty()){
        int u = q.top().se;
        q.pop();
        if(vt[u]) continue;
        vt[u] = 1;
        for(int i : g[u]){
            if(dt[i] > dt[u] + 1ll * (h[i] - h[u]) * (h[i] - h[u])){
                dt[i] = dt[u] + 1ll * (h[i] - h[u]) * (h[i] - h[u]);
                q.push({dt[i], i});
            }
        }
    }

    // for(int i = 1; i <= n; ++i) cout << ds[i] << ' ';
    // db();
    // for(int i = 1; i <= n; ++i) cout << dt[i] << ' ';
    // db();

    
    ld mod = 0.0;
    for(int x = 1; x <= n; ++x){
        seg *rt_s = new seg(0, N);
        seg *rt_t = new seg(0, N);
        for(int i : g[x]){
            ll tmp = 2 * ds[i] + rt_s -> query(h[i]) + h[i] * h[i];
            ans = min(ans, tmp);


            ll a = -2 * h[i];
            ll b = h[i] * h[i] + 2 * dt[i];
            //db(a, b);
            rt_s -> insert({a, b});
        }

        for(int i : g[x]){
            ll tmp = 2 * dt[i] + rt_t -> query(h[i]) + h[i] * h[i];
            // db(x, i, res.se, res.fi, tmp);
            ans = min(ans, tmp);


            ll a = -2 * h[i];
            ll b = h[i] * h[i] + 2 * ds[i];
            // db(a, b);
            rt_t -> insert({a, b});
        }
    }
    
    mod = ans;
    mod *= 0.5;
    cout << fixed << setprecision(8) << mod << '\n';

}