l,r=map(int,input().split())
lis=[]
for i in range(l,r+1):
    lis.append((i,i))
    if i < r+1:
        lis.append((i,i+1))
n= r+r

sosu=[i for i in range(n+1) ]
for i in range(4,n+1,2):
    sosu[i]=0
for j in range(3,int(n**0.5)+1,2):
    for k in range(j,n//j+1,2):
        sosu[j*k]=0
        
set_p=set(sosu[2:])
ans=0

for i,j in lis:
    if i == j :
        if i in set_p:
            ans+=1            
    else:
        if i+j in set_p:
            ans+=1
print(ans)