import itertools
from collections import defaultdict

L, R = map(int, input().split())


def primes(L, R):
    is_prime = [True] * (R + 1)
    is_prime[0] = False
    is_prime[1] = False
    for i in range(L, int(R ** 0.5) + 1):
        if not is_prime[i]:
            continue
        for j in range(i * 2, R + 1, i):
            is_prime[j] = False
    return (i for i in range(L, R + 1) if is_prime[i])


P = tuple(primes(L, R))
d = defaultdict(lambda: float('inf'))
for p1, p2 in itertools.combinations(P, 2):
    d[p2] = min(d[p2], p2 % p1)
print(sum(d.values()))