def gcd(a, b):
    while b:
        a, b = b, a%b
    return a
    
A = int(raw_input())
B = int(raw_input())

i=A
j=B
max=-1
fA=0
fB=0

while (i<=B):
    while (j>=A):
        if gcd(i,j)==1:
            if (abs(j-i)+1) > max: 
                fA=i
                fb=j
                max=abs(j-i)+1
        j=j-1
    i=i+1
    
print max