import java.util.Scanner;

public class ShopAndShip {
    public static int minIndex(int[] d, byte[] c) {
        int min = 100000;
        int index = 0;
        for (int i = 0; i < d.length; i++) {
            if (c[i] == 0 && d[i] < min) {
                min = d[i];
                index = i;
            }
        }
        return index;
    }

    public static void main (String[] args) {
        Scanner s = new Scanner(System.in);
        int n = s.nextInt();
        int[][] graph = new int[n][n];
        int[] dist = new int[n];
        int[] cost = new int[n];
        byte[] checked = new byte[n];
        int t = s.nextInt();
        for (int i = 0; i < n; i++) {
            cost[i] = -1;
        }
        for (int i = 0; i < t; i++) {
            int x = s.nextInt() - 1;
            int y = s.nextInt() - 1;
            int c = s.nextInt();
            graph[x][y] = c;
            graph[y][x] = c;
        }
        int k = s.nextInt();
        for (int i = 0; i < k; i++) {
            int z = s.nextInt() - 1;
            int p = s.nextInt();
            cost[z] = p;
        }
        int d = s.nextInt() - 1;
        for (int i = 0; i < n; i++) {
            if (i != d)
                dist[i] = 100000;
            else
                dist[i] = 0;
        }
        for (int i = 0; i < n; i++) {
            int v = minIndex(dist, checked);
            for (int u = 0; u < n; u++) {
                if (graph[u][v] != 0 && dist[v] + graph[u][v] < dist[u])
                    dist[u] = dist[v] + graph[u][v];
            }
            checked[v] = 1;
        }
        int min = 100000;
        for (int i = 0; i < n; i++) {
            if (cost[i] != -1) {
                cost[i] += dist[i];
                if (cost[i] < min)
                    min = cost[i];
            }
        }
        System.out.println(min);
    }
}