easyctf-2017/library/grader.py
2017-03-11 00:34:41 -06:00

39 lines
846 B
Python

x = input() + 1
mat = [[0,0,1,0],[0,1,0,1],[1,0,2,0],[0,2,0,1]]
mod = 10**9+7
def egcd(a, b):
if a == 0:
return (b, 0, 1)
else:
g, y, x = egcd(b % a, a)
return (g, x - (b // a) * y, y)
def modinv(a, m):
g, x, y = egcd(a, m)
if g != 1:
raise Exception('modular inverse does not exist')
else:
return x % m
def matmult(mtx_a, mtx_b, mod):
tpos_b = zip( *mtx_b)
rtn = [[ sum( ea*eb for ea,eb in zip(a,b))%mod for b in tpos_b] for a in mtx_a]
return rtn
def trace(A):
return sum(A[j][j] for j in range(len(A)))
def matpow(A, p):
ret = A
for bit in bin(p)[3:]:
ret = matmult(ret, ret, mod)
if bit=='1':
ret = matmult(ret, A, mod)
return ret
inv4 = modinv(4, mod)
ans = trace(matpow(mat, x))%mod
ans = (ans * inv4)% mod
print ans