#include <iostream>
#include <algorithm>
#include <queue>
#include <stack>

using namespace std;

int main() {
    int n, m; cin >> n >> m;
    vector<int> joy(n);
    vector<double> value(n);
    vector<vector<int>> adj(n);
    vector<vector<int>> prev(n);
    vector<int> adjct(n);
    queue<int> q;
    stack<int> s;

    for(int i = 0; i < n; i++)
        cin >> joy[i];
    for(int i = 0; i < m; i++) {
        int s, t;
        cin >> s >> t;
        adj[s].push_back(t);
        prev[t].push_back(s);
    }

    for(int i = 0; i < n; i++) {
        adjct[i] = adj[i].size();
        if(adjct[i] == 0)
            s.push(i);
    }


    while(!s.empty()) {
        int cur = s.top(); s.pop();
        q.push(cur);

        for(int nb : prev[cur]) {
            adjct[nb]--;
            if(adjct[nb] == 0) {
                s.push(nb);
            }
        }
    }

    while(!q.empty()) {
        int cur = q.front(); q.pop();

        // This assumes that you do not fill yourself up at your final stall.
        value[cur] = joy[cur];
        for(int nb : adj[cur]) {
            value[cur] = max(value[cur], joy[cur] + value[nb]/2.0);
            value[cur] = max(value[cur], value[nb]);
        }
    }

    printf("%.10lf\n", value[0]);


    return 0;
}