I am doing Project Euler problem #8:The four adjacent digits in the 1000-digit number that have the  greatest product are 9  9  8  9 = 5832.7316717653133062491922511967442657474235534919493496983520312774506326239578318016984801869478851843858615607891129494954595017379583319528532088055111254069874715852386305071569329096329522744304355766896648950445244523161731856403098711121722383113622298934233803081353362766142828064444866452387493035890729629049156044077239071381051585930796086670172427121883998797908792274921901699720888093776657273330010533678812202354218097512545405947522435258490771167055601360483958644670632441572215539753697817977846174064955149290862569321978468622482839722413756570560574902614079729686524145351004748216637048440319989000889524345065854122758866688116427171479924442928230863465674813919123162824586178664583591245665294765456828489128831426076900422421902267105562632111110937054421750694165896040807198403850962455444362981230987879927244284909188845801561660979191338754992005240636899125607176060588611646710940507754100225698315520005593572972571636269561882670428252483600823257530420752963450Find the thirteen adjacent digits in the 1000-digit number that have  the greatest product. What is the value of this product?Here is my code to this problem:L = []L.append(73167176531330624919225119674426574742355349194934)L.append(96983520312774506326239578318016984801869478851843)L.append(85861560789112949495459501737958331952853208805511)L.append(12540698747158523863050715693290963295227443043557)L.append(66896648950445244523161731856403098711121722383113)L.append(62229893423380308135336276614282806444486645238749)L.append(30358907296290491560440772390713810515859307960866)L.append(70172427121883998797908792274921901699720888093776)L.append(65727333001053367881220235421809751254540594752243)L.append(52584907711670556013604839586446706324415722155397)L.append(53697817977846174064955149290862569321978468622482)L.append(83972241375657056057490261407972968652414535100474)L.append(82166370484403199890008895243450658541227588666881)L.append(16427171479924442928230863465674813919123162824586)L.append(17866458359124566529476545682848912883142607690042)L.append(24219022671055626321111109370544217506941658960408)L.append(07198403850962455444362981230987879927244284909188)L.append(84580156166097919133875499200524063689912560717606) L.append(05886116467109405077541002256983155200055935729725)L.append(71636269561882670428252483600823257530420752963450)M = []for s in L:    for i in list(s): M.append(i)greatest_product = 0for i in range(len(M)):    product = 1    substring = M[i:i+13]    for digit in substring:         product*=int(digit)    if product > greatest_product:        greatest_product = productprint greatest_productThe code works, but how would I improve the efficiency?
Efficiency on Project Euler Problem #8
python;programming challenge