#include <cmath>
#include <cstdio>
#include <vector>
#include <iostream>
#include <queue>
#include <algorithm>
using namespace std;
vector<vector<int>> graph;
vector<int> path;
int distance(int source ,int target){
    vector<bool> used;
    used.resize(graph.size());
    queue<pair<int,int>> q;
    used[path[source]] = true;
    q.push({path[source], 0});
    for(int i=source;i<path.size();i++){
        used[path[i]] = true;
    }
    int finalDistance = -1;
    bool stop = false;
    while(!q.empty()){
        if(stop) break;
        auto current = q.front();
        q.pop();
        int distance = current.second;
        int element = current.first;
        for(auto n: graph[element]){
            if(n == path[target]){
                finalDistance = distance+1;
                stop = true;
                break;
            }
            if(!used[n]){
                used[n] = true;
                q.push({n, distance+1});
            }
        }
    }
    return finalDistance;
}
int main() {
    int n,m;
    scanf("%d%d", &n, &m);
    int u,v;
    graph.resize(n);
    for(int i=0;i<m;i++){
        scanf("%d%d", &u, &v);
        graph[u].push_back(v);
    }
    int k;
    scanf("%d", &k);
    path.resize(k);
    for(int i=0;i<k;i++){
        scanf("%d", &path[i]);
    }
    int sum = 0;
    int currentD = 0;
    for(int i=0;i<k-1;i++){
        currentD = distance(i,i+1);
        if(currentD==-1){
            cout<<-1;
            return 0;
        }
        sum+=currentD;
    }
    cout<<sum;
    return 0;
}