import java.util.ArrayList;
import java.util.List;
import java.util.Scanner;

public class PizzaWorld_HarryACC {

	static double[] enjoyment;
	static List<List<Integer>> connections;

	static double[] maxima;

	public static void main(String[] args) {
		Scanner sc = new Scanner(System.in);

		int n = sc.nextInt(); int m = sc.nextInt();

		enjoyment = new double[n];
		connections = new ArrayList<List<Integer>>(n);

		maxima = new double[n];

		for(int i = 0; i < n; i++)
			connections.add(new ArrayList<Integer>());


		for(int i = 0; i < n; i++)
			enjoyment[i] = sc.nextDouble();
		for(int j = 0; j < m; j++)
		{
			int s = sc.nextInt(); int t = sc.nextInt();
			connections.get(s).add(t);
		}
		for(int i = 0; i < n; i++)
			maxima[i] = -1; // Initialization at a negative value to check whether we've calculated this place already.

		System.out.println(dp(0));

	}

	public static double dp(int s)
	{
		if(maxima[s] != -1) return maxima[s];

		maxima[s] = enjoyment[s];

		for(int t: connections.get(s))
		{
			double dpt = dp(t);
			if(dpt > maxima[s]) maxima[s] = dpt;

			double value = enjoyment[s] + 0.5 * dpt;
			if(value > maxima[s]) maxima[s] = value;
		}

		return maxima[s];


	}

}