#include <bits/stdc++.h>

#define forn(i, n) for(int i = 0; i < (int)n; ++i)
#define for1(i, n) for(int i = 1; i <= (int)n; ++i)
#define fore(i, l, r) for(int i = (int)l; i <= (int)r; ++i)
#define all(x) x.begin(), x.end()
#define el '\n'
#define d(x) cerr << #x << ' ' << x << el

#define sz(a) (int) a.size()
#define pb push_back
using namespace std;

typedef long long ll;
typedef vector<int> vi;
typedef array<int, 4> a4;
typedef pair<int,int> ii;
typedef tuple<ll,int,int> iii;
const int nax = 2e5;
// const int nax = 2e3;
vi g[nax];
ll dist[3][nax];
const ll inf = 1e17;
void solve(){
    int n,m,k,d;
    cin>>n>>m>>k>>d;
    --k;
    forn(i,m){
        int u,v;
        cin>>u>>v;
        --u,--v;
        g[u].pb(v), g[v].pb(u);
    }

    queue<int> q;
    q.push(k);
    dist[0][k] = 0;
    vi visited(n);
    visited[k] = 1;
    while(!q.empty()){
        int siz = sz(q);
        while(siz--){
            int u = q.front();
            q.pop();
            ll curDist = dist[0][u];
            for(int v : g[u]){
                if(!visited[v])
                {
                    visited[v] = 1;
                    q.push(v);
                    dist[0][v] = curDist + 1;
                }
            }
        }
    }
    priority_queue<iii, vector<iii>, greater<iii>> qq;
    qq.push({0,0,1});
    vector<vector<ll>> curDist(2, vector<ll>(n, inf));
    curDist[1][0] = 0;
    while(!qq.empty()){
        int siz = sz(qq);
        while(siz--){
            auto [distU, u, same] = qq.top();
            // d(distU), d(u), d(same);
            qq.pop();
            if(distU > curDist[same][u]) continue;

            for(int v : g[u]){
                // d(v);
                bool newSame = (dist[0][v] < d) && same;
                // d(newSame);
                ll cost = d;
                if(newSame) cost = d - dist[0][v];
                if(distU + cost < curDist[newSame][v]){
                    curDist[newSame][v] = distU + cost;
                    qq.push({curDist[newSame][v], v, newSame});
                }
            }
        }
    }
    ll ans = min(curDist[0][n-1], curDist[1][n-1]);
    cout<<ans<<el;
}
int main(){
    ios_base::sync_with_stdio(false);
    cin.tie(NULL);
    int tt = 1;
    // cin>>tt;
    while(tt--){
        solve();
    }
}