/* -*- coding: utf-8 -*-
 *
 * 2018.cc:  No.2018 X-Y-X - yukicoder
 */

#include<cstdio>
#include<algorithm>
 
using namespace std;

/* constant */

const int MAX_N = 200000;

/* typedef */

typedef long long ll;

/* global variables */

char s[MAX_N + 4], t[MAX_N + 4];
int as[MAX_N], bs[MAX_N], ps[MAX_N], qs[MAX_N];

/* subroutines */

int s2v(int n, char s[], int as[], int ps[]) {
  int oc = 0;
  for (int i = 0; i + 1 < n; i++) {
    int a0 = s[i] - 'A', a1 = s[i + 1] - 'A';
    as[i] = a0 ^ a1 ^ (i & 1);
    if (as[i]) ps[oc++] = i;
  }
  return oc;
}

/* main */

int main() {
  int n;
  scanf("%d%s%s", &n, s, t);

  int m = n - 1;
  int ac = s2v(n, s, as, ps);
  int bc = s2v(n, t, bs, qs);
  //for (int i = 0; i < m; i++) printf("%d", as[i]); putchar('\n');
  //for (int i = 0; i < m; i++) printf("%d", bs[i]); putchar('\n');

  if (ac != bc) { puts("-1"); return 0; }

  ll sum = 0;
  for (int i = 0; i < ac; i++) sum += abs(qs[i] - ps[i]);

  printf("%lld\n", sum);
  return 0;
}