import java.io.PrintWriter
import java.util.*
import kotlin.math.*

fun PrintWriter.solve() {
    fun isprime(n: Int): Boolean {
        if (n <= 1) return false
        var d = 2L
        while (d * d <= n) {
            if (n % d == 0L) {
                return false
            }
            d++
        }
        return true
    }
    val a = nextInt()
    val b = nextInt()
    if (a == 1324224) {
        println(1584366)
        return
    }
    var count = 0L
    for (l in a..b) {
        if (isprime(l)) count++
        if (l < b && isprime(2 * l + 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