#include <bits/stdc++.h>

#define maxn 100005
#define ll long long
#define inf 1e18

using namespace std;

int n, m, s, t;
vector<int> g[maxn];

int h[maxn];
ll dis[2][maxn];

queue<int> q;
int in[maxn];

ll sqr(int x) {
	return 1ll * x * x;
}

void chk() {
	if (q.size() && dis[q.front()] > dis[q.back()]) swap(q.front(), q.back());
}

void spfa(int s, ll dis[]) {
	for (int i = 1; i <= n; i++) dis[i] = inf;
	dis[s] = 0;
	q.push(s), in[s] = 1;
	while (!q.empty()) {
		int u = q.front(); q.pop(); chk(); in[u] = 0;
		for (int v : g[u]) {
			if (dis[v] > dis[u] + sqr(h[u] - h[v])) {
				dis[v] = dis[u] + sqr(h[u] - h[v]);
				if (!in[v]) q.push(v), in[v] = 1, chk(); 
			}
		}
	}
//	for (int i = 1; i <= n; i++) cout<<dis[i]<<" "; puts("");
}

int main() {
	scanf("%d%d%d%d", &n, &m, &s, &t);
	for (int i = 1; i <= n; i++) scanf("%d", &h[i]), h[i] <<= 1;
	while (m--) {
		int x, y;
		scanf("%d%d", &x, &y);
		g[x].push_back(y);
		g[y].push_back(x);
	}
	spfa(s, dis[0]);
	spfa(t, dis[1]);
	ll ans = inf;
	for (int i = 1; i <= n; i++) {
		for (int x : g[i]) {
			for (int y : g[i]) {
//				cout<<i<<" "<<x<<" "<<y<<" "<<dis[0][x]<<" "<<dis[1][y]<<" "<<sqr(h[x] - h[y]) / 2<<endl;
				ans = min(ans, dis[0][x] + dis[1][y] + sqr(h[x] - h[y]) / 2);
			}
		}
	}
	printf("%Lf\n", (long double)ans / 4.0);
	return 0;
}