#include <iostream>
#include <cstring>
#include <cstdio>
#include <algorithm>
#include<set>
#include <cmath>
#include<sstream>
#include<queue>
#include<istream>
#include<vector>
#include<stdio.h>
#include<stack>
#include<deque>
#include<bitset>
#include<string>
#include<cstdio>
#include<map>
#include<iterator>
#include<iomanip>
#define prfs(x) printf("%s\n",x.c_str())
#define prfi(x) printf("%d\n",x);
#define sfi(X) scanf("%d",&X);
#define lop(i,n) for (int i = 0 ; i < n ;i++)
#define blop(i,n) for(int i = n-1 ; i>=0;i--)
#define M_P make_pair
#define All(X) (X).begin(),(X).end()
#define SZ(n) (int)(n).size()
#define voi vector<int>
#define Allr(X) (X).rbegin(),(X).rend()
#define vos vector<string>
#define vob vector<bool>
#define vovi vector<vector<int > >
#define vob vector<bool>
#define ll long long
using namespace std;
vector<vector<int> > g;
int X, Z;
double Y;
struct river {
double capacity =10000000.0;
double initial = 0.0;
};
map<int, river> MAP;
int main()
{
int N, K;scanf("%d%d", &N, &K);
for (int i = 0;i < N; i++)
{
double cap;
double inti;
cin >> cap >> inti;
MAP[i].capacity = cap, MAP[i].initial = inti;
}
g.resize(N);
for (int j = 0;j < K; j++)
{
int From, To;
scanf("%d%d", &From, &To);
From--, To--;
g[From].push_back(To);
}
cin >> X >> Y >> Z;
queue < pair<int,double> > q;
q.push(M_P(X-1, Y));
while (!q.empty())
{
int u = q.front().first;
double z = q.front().second;
q.pop();
if (MAP[u].capacity - MAP[u].initial < z)
{
z -= (MAP[u].capacity - MAP[u].initial);
MAP[u].initial = MAP[u].capacity;
}
else
{
MAP[u].initial += z;
continue;
}
for (int i = 0; i < g[u].size(); i++)
{
q.push(M_P(g[u][i],z/g[u].size()));
}
}
cout << MAP[Z - 1].initial<< endl;
return 0;
}
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