//#include <bits/stdc++.h>
#include <iostream>
//#include <fstream>
//ifstream f("fisier.in");
using namespace std;

int main() {
    int nr,i,j,tatal,fiul,q,x,k,s;
    double raspuns;
    s = 0;
    cin>> nr;
    int** matrix = new int* [nr];
    for ( i = 0; i < nr; ++i)
        matrix[i] = new int[nr];
    for (i = 0; i < nr; i++)
        for (j = 0; j < nr; j++)
            matrix[i][j] = 0;
    for (j = 0; j < nr - 1; j++) {
        cin >> tatal >> fiul;
        matrix[0][fiul-1] = tatal;
    }
    for (i = 1; i < nr; i++) {
        for (j = 0; j < nr; j++) {
            if(matrix[i-1][j]!=0)
                matrix[i][j] = matrix[0][matrix[i - 1][j]-1];
        }
    }
    cin >> q;
    for (i = 0; i < q; i++) {
        cin >> x >> k;
        s += matrix[k-1][x-1];
    }
    /*cout << s << endl;
    for (i = 0; i < nr; i++) {
        for (j = 0; j < nr; j++)
            cout<<matrix[i][j]<<" " ;
        cout << endl;
    }*/
    raspuns = s / (double)q;
    cout << /*setprecision(3) <<*/ raspuns;
    return 0;
}