#include <iostream>
#include <vector>
using namespace std;
using ld = long double;
ld dfs(int u, const vector<vector<int>>& g, const vector<ld>&c, vector<ld>&e){
	e[u] = 0;
	for(auto v : g[u]) e[u] = max(e[u], dfs(v,g,c,e));
	return e[u] = max(e[u], c[u] + e[u]/2);
}
int main(){
	int n,m,u,v; cin >> n >> m;
	vector<ld> c(n), e(n, -1);
	vector<vector<int>> g(n);
	for(auto &x : c) cin >> x;
	while(m--) cin >> u >> v, g[u].push_back(v);
	cout << dfs(0, g, c, e) << endl;
}