import sys, random
input = lambda : sys.stdin.readline().rstrip()


write = lambda x: sys.stdout.write(x+"\n"); writef = lambda x: print("{:.12f}".format(x))
debug = lambda x: sys.stderr.write(x+"\n")
YES="Yes"; NO="No"; pans = lambda v: print(YES if v else NO); INF=10**18
LI = lambda : list(map(int, input().split())); II=lambda : int(input()); SI=lambda : [ord(c)-ord("a") for c in input()]
def debug(_l_):
    for s in _l_.split():
        print(f"{s}={eval(s)}", end=" ")
    print()
def dlist(*l, fill=0):
    if len(l)==1:
        return [fill]*l[0]
    ll = l[1:]
    return [dlist(*ll, fill=fill) for _ in range(l[0])]

n = int(input())
s = [c=="B" for c in input()]
t = [c=="B" for c in input()]
if s[0]==t[0] and s[n-1]==t[n-1]:
    s = [s[i]^s[i+1] for i in range(n-1)]
    t = [t[i]^t[i+1] for i in range(n-1)]
    n -= 1
    for i in range(1,n,2):
        s[i] ^= 1
        t[i] ^= 1
    if sum(s)!=sum(t):
        ans = -1
    else:
        ans = 0
        sind = [i for i in range(n) if s[i]]
        tind = [i for i in range(n) if t[i]]
        for i in range(len(sind)):
            ans += abs(sind[i]-tind[i])
else:
    ans = -1
if ans==-1:
    print(-1)
else:
    print(ans)