import java.io.PrintWriter
import java.util.*

fun PrintWriter.solve() {
    val l = nextInt()
    val r = nextInt()
    val size = 2 * r
    val isprime = BooleanArray(size) { true }
    for (p in 2 until size) {
        if (!isprime[p]) continue
        var i = 2
        while (p * i < size) {
            isprime[p * i] = false
            i++
        }
    }
    isprime[1] = false
    var count = 0
    for (i in l..r) {
        if (isprime[i]) count++
    }
    for (i in l until r) {
        if (isprime[2 * i + 1]) count++
    }
    println(count)
}

fun main() {
    val writer = PrintWriter(System.out, false)
    writer.solve()
    writer.flush()
}

// region Scanner
private var st = StringTokenizer("")
private val br = System.`in`.bufferedReader()

fun next(): String {
    while (!st.hasMoreTokens()) st = StringTokenizer(br.readLine())
    return st.nextToken()
}

fun nextInt() = next().toInt()
fun nextLong() = next().toLong()
fun nextLine() = br.readLine()
fun nextDouble() = next().toDouble()
// endregion