C

#include <bits/stdc++.h>
#define fi first
#define se second
using namespace std;
using i64 = long long;
using ii = pair<i64, i64>;
const int INF = 0x3f3f3f3f;

int n, m, k;
vector<int> wormholes;
vector<vector<int>> gr;

void bfs(int s, vector<int> &d) {
  queue<int> q;
  d[s] = 0;
  q.push(s);
  while (!q.empty()) {
    int u = q.front(); q.pop();
    for (int to : gr[u]) if (d[to] == INF) {
      d[to] = d[u] + 1;
      q.push(to);
    }
  }
}

ii normalize(ii a) {
  i64 g = __gcd(a.fi, a.se);
  a.fi /= g;
  a.se /= g;
  return a;
}

ii sum(ii a, ii b) {
  a = normalize(a);
  b = normalize(b);
  i64 lcm = a.se / __gcd(a.se, b.se) * b.se;
  a.fi *= (lcm / a.se);
  b.fi *= (lcm / b.se);
  a.se = b.se = lcm;
  a.fi += b.fi;
  return a;
}

ii min(ii a, ii b) {
  return a.fi * b.se < b.fi * a.se ? a : b;
}

int main() {
  cin.tie(0)->sync_with_stdio(0);
  cin >> n >> m >> k;
  gr.assign(n, vector<int>());

  for (int i = 0; i < k; ++i) {
    int x; cin >> x, --x;
    wormholes.push_back(x);
  }

  for (int i = 0; i < m; ++i) {
    int a, b; cin >> a >> b, --a, --b;
    gr[a].push_back(b);
    gr[b].push_back(a);
  }

  vector<int> dist[2];
  dist[0].assign(n, INF);
  dist[1].assign(n, INF);

  bfs(0, dist[0]);
  bfs(n-1, dist[1]);

  i64 S = 0;
  for (int s : wormholes) {
    S += dist[1][s];
  }

  ii ans = {dist[0][n-1], 1};

  for (int s : wormholes) {
    ans = min(ans, sum({dist[0][s], 1}, {S - dist[1][s], k - 1}));
  }

  ans = normalize(ans);
  cout << ans.fi << '/' << ans.se << '\n';

  return 0;
}