module Graphical
	require 'set'

	def reachable_from(s, max_depth = nil)
		reached = Set[s]
		queue = [s]
		depth = 0
		loop {
			break if max_depth && depth >= max_depth
			next_queue = []
			until queue.empty?
				u = queue.shift
				each_out_connection_of(u){|v, _|
					next if reached.include?(v)
					reached << v
					next_queue << v
				}
			end
			break if next_queue.empty?
			queue = next_queue
			depth += 1
		}
		return reached
	end

	def bfs(start)
		raise "start must be Enumerable" if !start.kind_of?(Enumerable)
		reached = Set[*start.to_a]
		queue = []
		start.each{|s| queue << s}
		dist = {}
		loop.with_index {|_, depth|
			next_queue = []
			until queue.empty?
				u = queue.shift
				return depth if block_given? && yield(u, depth)
				dist[u] = depth
				each_out_connection_of(u){|v, _|
					next if reached.include?(v)
					reached << v
					next_queue << v
				}
			end
			break if next_queue.empty?
			queue = next_queue
		}
		return dist
	end

	def dijkstra(start)
		raise "start must be Enumerable" if !start.kind_of?(Enumerable)
		h = Heap.new
		reached = Set[*start.to_a]
		start.each{|s| h.add(s, 0)}

		dist = {}
		until h.empty?
			u, key = h.del_min
			return key if block_given? && yield(u, key)
			dist[u] = key
			each_out_connection_of(u){|v, l|
				d = dist[u] + l
				if reached.include?(v)
					h.update_key(v, d)
				else
					h.add(v, d)
					reached << v
				end
			}
		end
		block_given? ? nil : dist
	end

	def bidir_dijkstra(s, t)
		f_dist = {}
		b_dist = {}

		f_heap = Heap.new
		b_heap = Heap.new

		f_heap.add(s, 0)
		b_heap.add(t, 0)

		f_reached = Set[s]
		b_reached = Set[t]

		f_current_dist = b_current_dist = 0
		best = Float::INFINITY

		loop {
			if !f_heap.empty?
				u, key = f_heap.del_min
				f_dist[u] = key
				f_current_dist = key
				each_out_connection_of(u){|v, l|
					d = f_dist[u] + l
					if b_dist[v]
						path_len = d + b_dist[v]
						best = path_len if path_len < best
					end
					if f_reached.include?(v)
						f_heap.update_key(v, d)
					else
						f_heap.add(v, d)
						f_reached << v
					end
				}
			end

			if !b_heap.empty?
				u, key = b_heap.del_min
				b_dist[u] = key
				b_current_dist = key
				each_in_connection_of(u){|v, l|
					d = b_dist[u] + l
					if f_dist[v]
						path_len = d + f_dist[v]
						best = path_len if path_len < best
					end
					if b_reached.include?(v)
						b_heap.update_key(v, d)
					else
						b_heap.add(v, d)
						b_reached << v
					end
				}
			end

			return best if f_current_dist + b_current_dist >= best
			return nil if f_heap.empty? && b_heap.empty?
		}
	end

	def a_star(start)
		raise "a_star need termination condition" unless block_given?
		h = Heap.new
		reached = Set[start]
		h.add(start, [a_star_heuristic(start), 0])

		dist = {}
		until h.empty?
			u, (e, d) = h.del_min
			return d if yield(u, dist)
			dist[u] = d
			each_out_connection_of(u){|v, l|
				d = dist[u] + l
				e = d + a_star_heuristic(v)
				if reached.include?(v)
					h.update_key(v, [e, d])
				else
					h.add(v, [e, d])
					reached << v
				end
			}
		end
		block_given? ? nil : dist
	end

	#make_variations :bidir_dijkstra

	def prim(start)
		h = Heap.new
		h.add(start, 0)
		reached = Set[start]
		tree_len = 0

		until h.empty?
			u, key = h.del_min
			tree_len += key

			each_connection_of(u){|v, d|
				if reached.include?(v)
					h.update_key(v, d)
				else
					h.add(v, d)
					reached << v
				end
			}
		end
		tree_len
	end

	def each_layer_from(start)
		raise "start must be Enumerable" if !start.kind_of?(Enumerable)
		return enum_for(__method__, start) unless block_given?
		reached = Set[*start]
		current_layer = start

		loop {
			yield current_layer
			next_layer = []
			current_layer.each{|u|
				each_out_connection_of(u){|v|
					next if reached.include?(v)
					reached << v
					next_layer << v
				}
			}	

			return if next_layer.empty?
			current_layer = next_layer
		}
	end
end

class Graph
	include Graphical

	def initialize(*options)
		@options = Set[*options]
	end

	def add_node(property = {})
		Node.new(property)
	end

	def add_edge(u, v, property = {})
		e = Edge.new(u, v, property)
		u.add_edge(e)
		v.add_edge(e)
		e
	end
	
	def del_edge(e)
		u, v = e.opposite.keys
		u.del_edge(e)
		v.del_edge(e)
		e
	end

	class Node
		def initialize(property)
			@property = property
			@edges = Set.new
		end

		def add_edge(e)
			@edges << e
		end

		def del_edge(e)
			@edges.delete(e)
		end

		def to_s
			@property[:name] || super
		end

		def each_edge
			@edges.each{|e| 
				yield e
			}
		end

		attr_reader :property
	end

	class Edge
		def initialize(u, v, property)
			@opposite = {u => v, v => u}
			@property = property
		end
		attr_reader :opposite, :property

		def to_s
			@property[:name] || super
		end

		def length
			@property[:length] || 1
		end
	end

	def each_out_connection_of(u)
		u.each_edge{|e|
			yield(e.opposite[u], e.length)
		}
	end

	alias :each_in_connection_of :each_out_connection_of
end

n, m, s, t = gets.split.map &:to_i

g = Graph.new
nodes = (0..n).map {|i| g.add_node({:name => i}) }
edges = []
m.times {
	i, j = gets.split.map &:to_i 
	edges << g.add_edge(nodes[i], nodes[j])
}

f_layers = g.each_layer_from([nodes[s]]).to_a
f_dist = g.bfs([nodes[s]])
b_layers = g.each_layer_from([nodes[t]]).to_a
b_dist = g.bfs([nodes[t]])
dst = f_dist[nodes[t]]

count = 0
(0..dst-2).each{|i|
	a = f_layers[i]
	next if !a
	a.sort_by!{|u| -b_dist[u]}

	b = b_layers[dst-2-i]
	next if !b
	b.sort_by{|u| f_dist[u]}

	k = 0	
	a.each{|u|
		k += 1 until k == b.size || b_dist[u] + f_dist[b[k]] >= dst-2
		count += b.size - k
	}
}
p count

Measured memory_bytes: 197524