#include <bits/stdc++.h>
using namespace std;
using ll = long long;
using db = double;
constexpr int mod = 998244353;
constexpr int N = 1e6 + 10;

void solve() {
    int n, m, k, d;
    cin >> n >> m >> k >> d;
    vector<vector<int>> edge(n + 1);
    for (int i = 1; i <= m; i++) {
        int u, v;
        cin >> u >> v;
        edge[u].push_back(v);
        edge[v].push_back(u);
    }
    auto calc = [&](ll len) -> ll {
        ll sum1 = 1ll * (len / d) * d * (d + 1) / 2;
        ll sum2 = 1ll * (len % d) * (d + d - len % d + 1) / 2;
        return sum1 + sum2;
    };
    auto bfs = [&](int s) -> vector<int> {
        vector<int> dis(n + 1, 1e9);
        queue<int> q;
        dis[s] = 0;
        q.push(s);
        while (!q.empty()) {
            int u = q.front(); q.pop();
            for (auto v : edge[u]) {
                if (dis[v] > dis[u] + 1) {
                    dis[v] = dis[u] + 1;
                    q.push(v);
                }
            }
        }
        return dis;
    };
    ll ans = 1e18;
    vector<int> disn = bfs(n);
    vector<int> disk = bfs(k);
    vector<ll> dis(n + 1, 1e18), vis(n + 1);
    priority_queue<pair<ll, int>> q;
    for (auto x : edge[1]) {
        if (disk[x] >= d) {
            ans = min(ans, calc(disn[x] + 1));
            // cerr << x << ' ' << disn[x] << ' ' << calc(disn[x]) << "\n";
        } else {
            dis[x] = d - disk[x];
            q.push({-dis[x], x});
        }
    }
    while (!q.empty()) {
        auto [d, u] = q.top(); q.pop();
        if (vis[u]) continue;
        vis[u] = true;
        for (auto v : edge[u]) {
            if (disk[v] >= d) {
                ans = min(ans, dis[u] + calc(disn[v] + 1));
                // cerr << u << ' ' << v << ' ' << dis[u] << ' ' << calc(disn[v]) << "\n";
            } else if (dis[v] > dis[u] + d - disk[v]) {
                dis[v] = dis[u] + d - disk[v];
                q.push({-dis[v], v});
            }
        }
    }
    cout << min(ans, dis[n]) << "\n";
}
int main() {
    ios::sync_with_stdio(false);
    cin.tie(nullptr);
    int tt = 1;
    // cin >> tt;
    while (tt--) {
        solve();
    }
    return 0;
}