import math
n,m = map(int,input().split())
s = 0
for a in range (n,m+1):
    f = True;
    for i in range (2,int(math.sqrt(a))+1):
        if a % i ==0:
            f = False;
            break;
    if f == True and a != 1:
            s += 1
print(s)

/**************************************************************
	Problem: 1063
	User: admin
	Language: Python
	Result: Accepted
	Time:149 ms
	Memory:34480 kb
****************************************************************/