#include<cstdio>
#include<vector>
using std::vector;
vector<int>g[100005];
int queue[100005];
int vis[100005];
int dis[100005];

int main(){
    int n,m,a,b;
    scanf("%d%d%d%d",&n,&m,&a,&b);
    for(int i = 1; i <= m; i++){
        int u,v;
        scanf("%d%d",&u,&v);
        v++;
        g[u].push_back(v);
        g[v].push_back(u);
    }
    b++;
    int h = 1, t = 0;
    for(int i = 1; i <= a; i++){
        vis[i] = 1;
        dis[i] = 0;
        queue[++t] = i;
    }
    while(h<=t){
        int u = queue[h++];
        for(int v: g[u]){
            if(!vis[v]){
                vis[v] = 1;
                dis[v] = dis[u]+1;
                queue[++t] = v;
            }
        }
    }
    int ans = -1;
    for(int i = b; i <= n+1; i++){
        if(vis[i]!=0){
            int offer = dis[i];
            if(ans==-1 || ans>offer) ans = offer;
        }
    }
    printf("%d\n",ans);
    return 0;
}