#include <bits/stdc++.h>

using LL = long long;

const int N = 2e5 + 7;

int n, a[N], ca, b[N], cb;
char s[N], t[N];

int main() {

  scanf("%d%s%s", &n, s + 1, t + 1);

  if(s[1] != t[1] || s[n] != t[n]) {
    puts("-1");
    return 0;
  }

  for(int i = 2; i <= n; i += 2) {
    s[i] = 'A' + 'B' - s[i];
    t[i] = 'A' + 'B' - t[i];
  }
  for(int i = 2; i <= n; ++i) {
    if(s[i] != s[i - 1])
      a[++ca] = i;
    if(t[i] != t[i - 1])
      b[++cb] = i;
  }

  if(ca != cb) {
    puts("-1");
    return 0;
  }

  LL ans = 0;
  for(int i = 1; i <= ca; ++i)
    ans += std::abs(a[i]- b[i]);

  printf("%lld\n", ans);

  return 0;
}