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


public class Greedy_Pizza {

	static double[] enjoyment;	
	static List<List<Integer>> connections;
	
	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);
		
		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);
		}
		
		
		
		System.out.println(greedy(0));
	}
	
	static double greedy(int s)
	{
		int best_path = -1;
		double best_value = enjoyment[s];
		int option = 0;
		for(int c: connections.get(s))
		{
			// Two options: eat both, skip first.
			double val1 = enjoyment[s] + 0.5 * enjoyment[c];
			double val2 = enjoyment[c];
			
			double max = Math.max(val1, val2);
			
			if(max >= best_value)
			{
				best_value = max;
				best_path = c;
				
				if(max == val1) option = 1;
				if(max == val2) option = 2;	
			}				
		}
		
		if(connections.get(s).isEmpty())
			return enjoyment[s];
		
		if(option == 0)
			return enjoyment[s] + 0.5 * greedy(connections.get(s).get(0)); // We've found nothing appetizing in the next step.
		
		if(option == 1)
			return enjoyment[s] + 0.5 * greedy(best_path);
		
		if(option == 2)
			return greedy(best_path);
		
		return 0;
	}
}