#include <bits/stdc++.h>
using namespace std;

#define all(x) begin(x), end(x)
#define sz(x) (ll)(x).size()

using ll = long long;
using ld = long double;

vector<vector<ll>> adj;

vector<ll> bfs(ll x) {
	vector<ll> dist(sz(adj), -1);
	dist[x] = 0;
	queue<ll> todo;
	todo.push(x);
	while (!todo.empty()) {
		ll c = todo.front();
		todo.pop();
		for (ll y : adj[c]) {
			if (dist[y] < 0) {
				dist[y] = dist[c] + 1;
				todo.push(y);
			}
		}
	}
	return dist;
}

int main() {
	ios_base::sync_with_stdio(false);
	cin.tie(nullptr);
	ll n, m, w;
	cin >> n >> m >> w;
	assert(w >= 2); //1 makes no sense, where would you end?
	vector<ll> holes(w);
	for (ll& x : holes) cin >> x, x--;
	vector<bool> isHole(n);
	for (ll x : holes) isHole[x] = true;
	adj.assign(n, {});
	for (ll i = 0; i < m; i++) {
		ll a, b;
		cin >> a >> b;
		a--;
		b--;
		adj[a].push_back(b);
		adj[b].push_back(a);
	}

	auto drev = bfs(n - 1);
	ll res = drev[0] * (w - 1);

	//bfs but prune after wormholes
	vector<ll> dadj(n, -1);
	vector<bool> pruned(n);
	dadj[0] = 0;
	queue<ll> todo;
	todo.push(0);
	while (!todo.empty()) {
		ll c = todo.front();
		if (c == n - 1) break;
		todo.pop();
		if (isHole[c] && !pruned[c]) {
			ll cur = 0;
			for (ll x : holes) {
				if (x != c) cur += dadj[c] + drev[x];
			}
			res = min(res, cur);
		}
		for (ll y : adj[c]) {
			if (dadj[y] < 0) {
				dadj[y] = dadj[c] + 1;
				todo.push(y);
			}
			if (dadj[y] == dadj[c] + 1) pruned[y] = pruned[y] | pruned[c] | isHole[c];
		}
	}
	ll g = gcd(res, w-1);
	cout << res/g << "/" << (w-1)/g << endl;
}