Buggy python submission (verdict: Runtime Error):
N, M = MAP()

S = input()
T = input()

if S[0] != T[0]:
	print(-1)
	exit()

G = gcd(N, M)

if G == 1:
	print(N*M)
	exit()

Fixed submission:
import sys, re
from collections import deque, defaultdict, Counter
from math import ceil, sqrt, hypot, factorial, pi, sin, cos, tan, asin, acos, atan, radians, degrees#, log2
from itertools import accumulate, permutations, combinations, combinations_with_replacement, product, groupby
from operator import itemgetter, mul
from copy import deepcopy
from string import ascii_lowercase, ascii_uppercase, digits
from bisect import bisect, bisect_left, insort, insort_left
from fractions import gcd
from heapq import heappush, heappop
from functools import reduce
def input(): return sys.stdin.readline().strip()
def INT(): return int(input())
def MAP(): return map(int, input().split())
def LIST(): return list(map(str, input().split()))
def ZIP(n): return zip(*(MAP() for _ in range(n)))
sys.setrecursionlimit(10 ** 9)
INF = float('inf')
mod = 10**9 + 7
#from decimal import *

N, M = MAP()

S = input()
T = input()

if S[0] != T[0]:
	print(-1)
	exit()

G = gcd(N, M)

n = N//G
m = M//G

for i in range(G):
	if S[::n][i] != T[::m][i]:
		print(-1)
		break
else:
	print(N*M//G)

Buggy python submission (verdict: Runtime Error):
import sys, re
from collections import deque, defaultdict, Counter
from math import ceil, sqrt, hypot, factorial, pi, sin, cos, tan, asin, acos, atan, radians, degrees#, log2
from itertools import accumulate, permutations, combinations, combinations_with_replacement, product, groupby
from operator import itemgetter, mul
from copy import deepcopy
from string import ascii_lowercase, ascii_uppercase, digits
from bisect import bisect, bisect_left, insort, insort_left
from fractions import gcd
from heapq import heappush, heappop
from functools import reduce
def input(): return sys.stdin.readline().strip()
def INT(): return int(input())
def MAP(): return map(int, input().split())
def LIST(): return list(map(str, input().split()))
def ZIP(n): return zip(*(MAP() for _ in range(n)))
sys.setrecursionlimit(10 ** 9)
INF = float('inf')
mod = 10**9 + 7
#from decimal import *

N, M = MAP()

S = input()
T = input()

L = N*M//gcd(N, M)
check = ["_"]*L
check[0] = S[0]

for i in range(N):
	check[L//N * i] = S[i]

for j in range(M):
	if check[L//M * j] != "_" and check[L//M * j] != T[j]:
		print(-1)
		break
else:
	print(L)

Fixed submission:
import sys, re
from collections import deque, defaultdict, Counter
from math import ceil, sqrt, hypot, factorial, pi, sin, cos, tan, asin, acos, atan, radians, degrees#, log2
from itertools import accumulate, permutations, combinations, combinations_with_replacement, product, groupby
from operator import itemgetter, mul
from copy import deepcopy
from string import ascii_lowercase, ascii_uppercase, digits
from bisect import bisect, bisect_left, insort, insort_left
from fractions import gcd
from heapq import heappush, heappop
from functools import reduce
def input(): return sys.stdin.readline().strip()
def INT(): return int(input())
def MAP(): return map(int, input().split())
def LIST(): return list(map(str, input().split()))
def ZIP(n): return zip(*(MAP() for _ in range(n)))
sys.setrecursionlimit(10 ** 9)
INF = float('inf')
mod = 10**9 + 7
#from decimal import *

N, M = MAP()

S = input()
T = input()

if S[0] != T[0]:
	print(-1)
	exit()

G = gcd(N, M)

n = N//G
m = M//G

for i in range(G):
	if S[::n][i] != T[::m][i]:
		print(-1)
		break
else:
	print(N*M//G)