l, r = map(int, input().split())
def Sieve_of_Eratosthenes(maxA):
    lst = [None]*(maxA+1)
    lst[0] = False
    lst[1] = False
    for i in range(2, maxA+1):
        if lst[i] is None:
            for g in range(i, maxA+1, i):
                if lst[g] is None:
                    lst[g] = False
            lst[i] = True
    return lst

lst = Sieve_of_Eratosthenes(2*r+1)
ans = int(lst[r])
for i in range(l, r):
    ans += lst[i]
    ans += lst[2*i+1]

print(ans)