Little town Nsk consists of n junctions connected by m bidirectional roads. Each road connects two distinct junctions and no two roads connect the same pair of junctions. It is possible to get from any junction to any other junction by these roads. The distance between two junctions is equal to the minimum possible number of roads on a path between them.

In order to improve the transportation system, the city council asks mayor to build one new road. The problem is that the mayor has just bought a wonderful new car and he really enjoys a ride from his home, located near junction s to work located near junction t. Thus, he wants to build a new road in such a way that the distance between these two junctions won't decrease. 

You are assigned a task to compute the number of pairs of junctions that are not connected by the road, such that if the new road between these two junctions is built the distance between s and t won't decrease.

Input

The firt line of the input contains integers n, m, s and t (2 ≤ n ≤ 1000, 1 ≤ m ≤ 1000, 1 ≤ s, t ≤ n, s ≠ t) — the number of junctions and the number of roads in Nsk, as well as the indices of junctions where mayors home and work are located respectively. The i-th of the following m lines contains two integers ui and vi (1 ≤ ui, vi ≤ n, ui ≠ vi), meaning that this road connects junctions ui and vi directly. It is guaranteed that there is a path between any two junctions and no two roads connect the same pair of junctions.

Output

Print one integer — the number of pairs of junctions not connected by a direct road, such that building a road between these two junctions won't decrease the distance between junctions s and t.

Examples

Input

5 4 1 5
1 2
2 3
3 4
4 5


Output

0


Input

5 4 3 5
1 2
2 3
3 4
4 5


Output

5


Input

5 6 1 5
1 2
1 3
1 4
4 5
3 5
2 5


Output

3
import java.io.*;
import java.util.*;
import java.math.*;
import java.math.BigInteger; 
//import javafx.util.*; 
public final class B
{    
	static StringBuilder ans=new StringBuilder();
	static FastReader in=new FastReader();
	static ArrayList<ArrayList<Integer>> g;	
	static long mod=1000000007;
	static int D1[],D2[],par[];
	static boolean set[],G[][];
	public static void main(String args[])throws IOException
	{  
		int N=i(),M=i(),st=i(),end=i();
		setGraph(N);
		D1=new int[N+1];
		D2=new int[N+1];
		Arrays.fill(D1, Integer.MAX_VALUE);
		Arrays.fill(D2, Integer.MAX_VALUE);
		G=new boolean[N+1][N+1];
		for(int i=0; i<M; i++)
		{
			int a=i(),b=i();
			g.get(a).add(b);
			g.get(b).add(a);
			G[a][b]=true;
			G[b][a]=true;
		}
		bfs(st,N,D1); bfs(end,N,D2);
		//print(D1); print(D2);
		int c=0;
		for(int a=1; a<N; a++)
		{
			for(int b=a+1; b<=N; b++)
			{
				if(D1[a]+D2[b]+1>=D1[end] && D1[b]+D2[a]+1>=D1[end] && G[a][b]==false)
				{
					c++;
				}
			}
		}System.out.println(c);
		
	}
	static void bfs(int n,int N,int D[])
	{
		Queue<Integer> q=new LinkedList<>();
		q.add(n);
		set=new boolean[N+1];
		//set[n]=true;
		D[n]=0;
		while(q.size()>0)
		{
			n=q.remove();
			set[n]=true;
			for(int c:g.get(n))
			{
				if(D[c]>D[n]+1)
				{
					q.add(c);
					D[c]=D[n]+1;
				}
			}
		}


	}
	static int find(int a)
	{
		if(par[a]<0)return a;
		return par[a]=find(par[a]);
	}
	static void union(int a,int b)
	{
		a=find(a);
		b=find(b);
		if(a!=b)
		{
			par[a]+=par[b]; //transfers the size
			par[b]=a;       //changes the parent
		}
	}
	static ArrayList<Integer> IND(int B[],HashMap<Integer,Integer> mp)
	{
		ArrayList<Integer> A=new ArrayList<>();
		for(int i:B)
		{
			if(mp.containsKey(i))
			{
				A.add(i);
				int f=mp.get(i)-1;
				if(f==0)
					mp.remove(i);
				else mp.put(i, f);		
			}
		}
		return A;
	}
	static HashMap<Integer,Integer> hash(int A[],int index)
	{
		HashMap<Integer,Integer> mp=new HashMap<Integer,Integer>();
		for(int i=index; i<A.length; i++)
		{
			int f=mp.getOrDefault(A[i], 0)+1;
			mp.put(A[i], f);
		}
		return mp;
	}
	static void swap(char A[],int a,int b)
	{
		char ch=A[a];
		A[a]=A[b];
		A[b]=ch;
	}

	static long lower_Bound(long A[],int low,int high, long x) 
	{ 
		if (low > high) 
			if (x >= A[high]) 
				return A[high]; 

		int mid = (low + high) / 2; 

		if (A[mid] == x) 
			return A[mid]; 

		if (mid > 0 && A[mid - 1] <= x && x < A[mid]) 
			return A[mid - 1]; 

		if (x < A[mid]) 
			return lower_Bound( A, low, mid - 1, x); 

		return lower_Bound(A, mid + 1, high, x); 
	} 
	static void sort(long[] a) //check for long
	{
		ArrayList<Long> l=new ArrayList<>();
		for (long i:a) l.add(i);
		Collections.sort(l);
		for (int i=0; i<a.length; i++) a[i]=l.get(i);
	}
	static void setGraph(int N)
	{
		//set=new boolean[N+1];
		//size=new int[N+1];
		//par=new int[N+1];
		g=new ArrayList<ArrayList<Integer>>();
		//tg=new ArrayList<ArrayList<Integer>>();
		for(int i=0; i<=N; i++)
		{
			g.add(new ArrayList<Integer>());
			//tg.add(new ArrayList<Integer>());
		}
	}

	static  long pow(long a,long b)
	{
		long mod=1000000007;
		long pow=1;
		long x=a;
		while(b!=0)
		{
			if((b&1)!=0)pow=(pow*x)%mod;
			x=(x*x)%mod;
			b/=2;
		}
		return pow;
	}
	static long toggleBits(long x)//one's complement || Toggle bits
	{
		int n=(int)(Math.floor(Math.log(x)/Math.log(2)))+1;

		return ((1<<n)-1)^x;
	}
	static int countBits(long a)
	{
		return (int)(Math.log(a)/Math.log(2)+1);
	}
	static void sort(int[] a) {
		ArrayList<Integer> l=new ArrayList<>();
		for (int i:a) l.add(i);
		Collections.sort(l);
		for (int i=0; i<a.length; i++) a[i]=l.get(i);
	}
	static boolean isPrime(long N)
	{
		if (N<=1)  return false; 
		if (N<=3)  return true; 
		if (N%2 == 0 || N%3 == 0) return false; 
		for (int i=5; i*i<=N; i=i+6) 
			if (N%i == 0 || N%(i+2) == 0) 
				return false; 
		return true; 
	}
	static long GCD(long a,long b) 
	{
		if(b==0)
		{
			return a;
		}
		else return GCD(b,a%b );
	}

	//Debugging Functions Starts

	static void print(char A[])
	{
		for(char c:A)System.out.print(c+" ");
		System.out.println();
	}
	static void print(boolean A[])
	{
		for(boolean c:A)System.out.print(c+" ");
		System.out.println();
	}
	static void print(int A[])
	{
		for(int a:A)System.out.print(a+" ");
		System.out.println();	
	}
	static void print(long A[])
	{
		for(long i:A)System.out.print(i+ " ");
		System.out.println();

	}
	static void print(ArrayList<Integer> A)
	{
		for(int a:A)System.out.print(a+" ");
		System.out.println();
	}

	//Debugging Functions END
	//----------------------
	//IO FUNCTIONS STARTS
	static HashMap<Integer,Integer> Hash(int A[])
	{
		HashMap<Integer,Integer> mp=new HashMap<>();
		for(int a:A)
		{
			int f=mp.getOrDefault(a,0)+1;
			mp.put(a, f);
		}
		return mp;
	}
	static int i()
	{
		return in.nextInt();
	}

	static long l()
	{
		return in.nextLong();
	}

	static int[] input(int N){
		int A[]=new int[N];
		for(int i=0; i<N; i++)
		{
			A[i]=in.nextInt();
		}
		return A;
	}

	static long[] inputLong(int N)     {
		long A[]=new long[N];
		for(int i=0; i<A.length; i++)A[i]=in.nextLong();
		return A;
	}

	//IO FUNCTIONS END

}


//Code For FastReader
//Code For FastReader
//Code For FastReader
//Code For FastReader
class FastReader
{
	BufferedReader br;
	StringTokenizer st;
	public FastReader()
	{
		br=new BufferedReader(new InputStreamReader(System.in));
	}

	String next()
	{
		while(st==null || !st.hasMoreElements())
		{
			try
			{
				st=new StringTokenizer(br.readLine());
			}
			catch(IOException e)
			{
				e.printStackTrace();
			}
		}
		return st.nextToken();
	}

	int nextInt()
	{
		return Integer.parseInt(next());
	}

	long nextLong()
	{
		return Long.parseLong(next());
	}

	double nextDouble()
	{
		return Double.parseDouble(next());
	}

	String nextLine()
	{
		String str="";
		try
		{
			str=br.readLine();
		}
		catch (IOException e)
		{
			e.printStackTrace();
		}
		return str;
	}

}