#include <bits/stdc++.h>
#define fi first
#define se second
using namespace std;

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

const ll INF = 1e18;

struct linecontainer{
    vector<pll>v;
    ll cnt=0;
    void clear() {
        v.clear(); cnt=0;
    }
    void insert(pll p) {
        while(v.size()>=2&&(__int128)(v.back().se-v[v.size()-2].se)*(v[v.size()-2].fi-p.fi) >= (__int128)(p.se-v[v.size()-2].se)*(v[v.size()-2].fi-v.back().fi) ) v.pop_back();
        v.push_back(p);
    }
    ll f(pll p,ll x) { return p.fi*x+p.se; }
    ll query(ll x)
    {
        while(cnt+1<v.size()&&f(v[cnt],x)>f(v[cnt+1],x)) cnt++;
        return f(v[cnt],x);
    }


}CHT;

ll dist[2][100005],n;
ll H[100005];
pll E[200005];
vector<ll>v[100005];
ll f(ll x,ll y) { return (H[x]-H[y])*(H[x]-H[y]); }
void dijk(ll s,ll t)
{
    fill(dist[t]+1,dist[t]+1+n,INF);
    dist[t][s]=0;
    priority_queue<pll,vector<pll>,greater<pll> > pq;
    pq.push({0,s});
    while(!pq.empty())
    {
        pll p=pq.top(); pq.pop();
        if(p.fi>dist[t][p.se]) continue;
        for(auto i:v[p.se])
        {
            pll q={p.fi+f(i,p.se),i};
            if(dist[t][q.se]>q.fi)
            {
                dist[t][q.se]=q.fi;
                pq.push(q);
            }
        }
    }
}
int main()
{
    ios::sync_with_stdio(0); cin.tie(0);
    ll m,S,T,ans=INF;
    cin>>n>>m>>S>>T;
    for(ll i=1;i<=n;i++) cin>>H[i], H[i]*=2;
    for(ll i=1;i<=m;i++)
    {
        cin>>E[i].fi>>E[i].se;
        v[E[i].fi].push_back(E[i].se); v[E[i].se].push_back(E[i].fi);
    }
    dijk(S,0); dijk(T,1);
    ans=dist[0][T];
    for(ll k=1;k<=n;k++)
    {
        vector<pll>line,target; CHT.clear();
        for(auto a:v[k]) line.push_back({-H[a],dist[1][a]+H[a]*H[a]/2}), target.push_back({H[a],dist[0][a]+H[a]*H[a]/2});
        sort(line.begin(),line.end(),greater<pll>()); sort(target.begin(),target.end());
        for(ll i=0;i<line.size();i++)
        {
            if(i+1<line.size()&&line[i].fi==line[i+1].fi) continue;
            CHT.insert(line[i]);
        }
        for(ll i=0;i<target.size();i++)
            ans=min(ans,CHT.query(target[i].fi)+target[i].se);
    }
    ans/=2;
    cout<<ans/2<<'.'<<ans%2*5;
    return 0;
}