L, R = gets.split(" ").map{|s| s.to_i}
n = 2*R-1

prime = Array.new(n+1, true)
prime[0] = prime[1] = false

2.upto(n) {|i|
	if prime[i] then
		k = 2
		while k * i <= n do
			prime[k*i] = false
			k += 1
		end
	end
}

cnt = 0
L.upto(R-1) {|i|
	cnt += 1 if prime[i]
	cnt += 1 if prime[2*i+1]
}
cnt += 1 if prime[R]

puts cnt