import sys
input = lambda :sys.stdin.readline()[:-1]
ni = lambda :int(input())
na = lambda :list(map(int,input().split()))
yes = lambda :print("yes");Yes = lambda :print("Yes");YES = lambda : print("YES")
no = lambda :print("no");No = lambda :print("No");NO = lambda : print("NO")
#######################################################################
n = ni()
def f(s):
    a = []
    c = 1
    for i in range(1,n):
        if s[i] != s[i-1]:
            a.append(i)
    return s[0], a

a,s = f(input())
b,t = f(input())
if a != b or len(s)!=len(t):
    print(-1)
    exit()
ans = 0
for i in range(len(s)):
    ans += abs(s[i]-t[i])

print(ans)