#include <algorithm>
#include <cstdio>
#include <vector>
using namespace std;

typedef long long lld;

int T = 0;

struct Vertex {
    vector< int > neighs;
    lld limit;
    double cur;
    int T_out, index;
};

inline bool operator <(const Vertex &a, const Vertex &b) {
    return a.T_out > b.T_out;
}

Vertex *nodes;

void DFS(int vertex) {
    for (unsigned i = 0; i < nodes[vertex].neighs.size(); i++)
        DFS(nodes[vertex].neighs[i]);
    nodes[vertex].T_out = ++T;
}

int main() {
    freopen("input.txt", "r", stdin); //

    int N, K;
    scanf("%d%d", &N, &K);
    nodes = new Vertex [N];
    for (int i = 0; i < N; i++) {
        scanf("%lld%lf", &nodes[i].limit, &nodes[i].cur);
        nodes[i].index = i;
    }
    for (int i = 0; i < K; i++) {
        int F, T;
        scanf("%d%d", &F, &T);
        F--;
        T--;
        nodes[F].neighs.push_back(T);
    }
    int start, target;
    lld start_adding;
    scanf("%d%lld%d", &start, &start_adding, &target);
    start--;
    target--;

    DFS(start);
    sort(nodes, nodes + N);

    nodes[0].cur += start_adding;
    for (int i = 0; i < target; i++) {
        if (nodes[i].cur > nodes[i].limit) {
            double adding = (nodes[i].cur - nodes[i].limit) / nodes[i].neighs.size();
            for (unsigned j = 0; j < nodes[i].neighs.size(); i++)
                nodes[nodes[i].neighs[j]].cur += adding;
        }
    }
    printf("%.4lf\n", min(nodes[target].cur, static_cast< double > (nodes[target].limit)));
    return 0;
}