class PrimeTable:
    def __init__(self, lim):
        self.isPrime = [True for _ in range(lim + 1)]
        self.isPrime[0] = self.isPrime[1] = False
        for i in range(2, lim):
            if self.isPrime[i]:
                for j in range(2 * i, lim + 1, i):
                    self.isPrime[j] = False
        self.acc = [0 for _ in range(lim + 1)]
        for i in range(2, lim + 1):
            self.acc[i] = self.acc[i - 1] + (1 if self.isPrime[i] else 0)
    def isPrime(self, val): return self.isPrime[val]
    def cntPrime(self, l, r): return self.acc[r] - self.acc[l - 1]

def main():
    L, R = map(int, input().split())
    pt = PrimeTable(2 * R)
    ans = pt.cntPrime(L, R) + pt.cntPrime(2 * L + 1, 2 * R)
    print(ans)    

main()