import sys

# sys.setrecursionlimit(200005)
int1 = lambda x: int(x)-1
pDB = lambda *x: print(*x, end="\n", file=sys.stderr)
p2D = lambda x: print(*x, sep="\n", end="\n\n", file=sys.stderr)
def II(): return int(sys.stdin.readline())
def LI(): return list(map(int, sys.stdin.readline().split()))
def LLI(rows_number): return [LI() for _ in range(rows_number)]
def LI1(): return list(map(int1, sys.stdin.readline().split()))
def LLI1(rows_number): return [LI1() for _ in range(rows_number)]
def SI(): return sys.stdin.readline().rstrip()
dij = [(0, 1), (-1, 0), (0, -1), (1, 0)]
# dij = [(0, 1), (-1, 0), (0, -1), (1, 0), (1, 1), (1, -1), (-1, 1), (-1, -1)]
inf = (1 << 63)-1
# inf = (1 << 31)-1
md = 10**9+7
# md = 998244353

from heapq import *

n = II()
s = SI()
t = SI()

if s[0] != t[0] or s[-1] != t[-1]:
    print(-1)
    exit()

aa = [(s[i] != s[i+1])*1 for i in range(n-1)]
bb = [(t[i] != t[i+1])*1 for i in range(n-1)]
# print(aa)
# print(bb)

hp = []
for i in range(n-2):
    if aa[i] == aa[i+1]: heappush(hp, i+1)

ans = 0
for i in range(n-1):
    if aa[i] == bb[i]: continue
    while 1:
        if not hp:
            print(-1)
            exit()
        j = heappop(hp)
        if j > i and aa[j-1] == aa[j]: break
    aa[j] ^= 1
    if j+1 < n-1 and aa[j] == aa[j+1]: heappush(hp, j+1)
    ans += j-i

print(ans)