import fractions

n, d = map(int,input().split())

if n == d:
    print(0)
    exit()

if d > n / 2:
    d = n - d

if fractions.gcd(n, d) == 1:
    ans = n - 1
else:
    if n % d == 0:
        ans = n // d - 1
    else:
        ans = n // d

print(ans)