# http://en.wikipedia.org/wiki/Curve25519 parameters
P = 2 ** 255 - 19
A = 486662
N = 7237005577332262213973186563042994240857116359379907606001950938285454250989

def expmod(b, e, m):
    if e == 0: return 1
    t = expmod(b, e / 2, m) ** 2 % m
    if e & 1: t = (t * b) % m
    return t

def inv(x):
    return expmod(x, P - 2, P)

# doubles a point on a montgomery curve (x-coordinate only representation)
# https://www.hyperelliptic.org/EFD/g1p/auto-montgom-xz.html#doubling-dbl-1987-m-3
def double(x1):
    xx1 = x1 * x1 % P
    x3 = (xx1 - 1) * (xx1 - 1) % P
    z3 = 4 * x1 * (xx1 * A * x1 + 1) % P
    return x3 * inv(z3) % P

def find(target, initial_point=9):
    assert 0 < target < P 
    assert 0 < initial_point < P
    x = initial_point
    i = 0
    while i < N:
        if x == target:
            return i 
        x = double(x)
        i += 1