//In the name of God
//#pragma GCC optimize("O2")
#include<bits/stdc++.h>
using namespace std;
typedef long long ll;
const ll MXN = 2e5 + 10;
const ll MXM = 6e5 + 10;
const ll INF = 1e9;
ll n, m, k, t, s[2], Src[4];
ll dis[4][MXN], Q[MXN];
vector<ll> adj[MXN];
bool vis[MXN];
void BFS(ll f, ll src){
fill(dis[f], dis[f] + MXN, INF);
memset(vis, 0, sizeof vis);
ll L = 0, R = 0; Src[f] = src;
dis[f][src] = 0, Q[R ++] = src, vis[src] = 1;
while(L < R){
ll u = Q[L ++];
for(auto v : adj[u]){
if(!vis[v]){
dis[f][v] = dis[f][u] + 1;
vis[v] = 1, Q[R ++] = v;
}
}
}
}
inline bool in_path(ll u, ll f0, ll f1){
ll s = Src[f0], e = Src[f1];
return (dis[f0][u] + dis[f1][u] == dis[f0][e]);
}
ll check(ll f, ll x){//tz check
if(dis[f][k] + x > dis[f][t]) return 1;
if(dis[f][k] + x < dis[f][t]) return 0;
return 2;
}
bool check(ll x){
ll ck0 = check(0, x), ck1 = check(1, x);
if(ck0 == 1 || ck1 == 1) return 0;
if(ck0 == 0 || ck1 == 0) return 1;
//return (!check(0, x)) && (!check(1, x));
ll tz = 0, km = 0;
for(int i = 1; i <= n; i ++){
if(in_path(i, 2, 0) && in_path(i, 2, 1)){
km = max(km, dis[2][i]);
}
if(in_path(i, 3, 0) && in_path(i, 3, 1)){
tz = max(tz, dis[3][i]);
}
}
return !(tz > km + x);
}
int main(){
ios::sync_with_stdio(0);cin.tie(0); cout.tie(0);
cin >> n >> m >> k >> t >> s[0] >> s[1];
for(int i = 1; i <= m; i ++){
ll u, v; cin >> u >> v;
adj[u].push_back(v), adj[v].push_back(u);
}
BFS(0, s[0]), BFS(1, s[1]);
BFS(2, k), BFS(3, t);
if(!check(0)) return cout << -1, 0;
ll low = 0, hig = n + 1;
while(hig - low > 1){
ll mid = (low + hig) / 2;
if(check(mid)) low = mid;
else hig = mid;
}
cout << low << '\n';
return 0;
}
// I was born to be making history.
// N.N
