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

n = II()
aa = [(c == "B")*1 for c in SI()]
bb = [(c == "B")*1 for c in SI()]

if aa[0] != bb[0] or aa[-1] != bb[-1]:
    print(-1)
    exit()

ans = 0
for i in range(n-1):
    if aa[i] == bb[i]: continue
    j = i
    while aa[j-1] != aa[j+1]:
        j += 1
        if j == n-1:
            print(-1)
            exit()
    while j >= i:
        aa[j] ^= 1
        ans += 1
        j -= 1
    # pDB(i,aa)
# pDB(bb)

print(ans)