#include<stdio.h>
#include<string.h>
#include<stdbool.h>

#define N 100010
#define M 1000010
#define INF 0x3f3f3f3f

int n, p, c, m;
int dist[N];
int que[N], head, tail;
bool inQue[N];
int h[N], cnt;

struct edge{
    int to;
    int next;
    int time;
}E[M*2];

void add(int v, int w, int time){
    E[++cnt].next = h[v];
    E[cnt].to = w;
    E[cnt].time = time;
    h[v] = cnt;
}

void spfa(){
    memset(dist, 0x3f, sizeof dist);
    dist[c] = 1;
    que[++tail] = c, inQue[c] = true;

    while(head != tail){
        int t = que[++head];
        tail %= N;
        inQue[t] = false;

        for(int i = h[t]; i != -1; i = E[i].next){
            int u = E[i].to;
            if(dist[u] > dist[t] + E[i].time){
                dist[u] = dist[t] + E[i].time;
                if(!inQue[u]){
                    que[++tail] = u;
                    tail %= N;
                    inQue[u] = true;
                }
            }
        }
    }
}

int main(){

    scanf("%d %d %d %d", &n, &p, &c, &m);
    memset(h, -1, sizeof h);

    for(int i = 1; i <= p; i++){
        int a, b;
        scanf("%d %d", &a, &b);
        add(a, b, 1), add(b, a, 1);
    }

    spfa();

    int max = -INF;
    for(int i = 1; i <= n; i++)
        if(dist[i] != INF && dist[i] > max)
            max = dist[i];

    printf("%d", max + m);

    return 0;
}