#include<bits/stdc++.h>
namespace {
#pragma GCC diagnostic push
#pragma GCC diagnostic ignored "-Wunused-function"
#include<atcoder/all>
#pragma GCC diagnostic pop
using namespace std;
using namespace atcoder;
#define rep(i,n) for(int i = 0; i < (int)(n); i++)
#define rrep(i,n) for(int i = (int)(n) - 1; i >= 0; i--)
#define all(x) begin(x), end(x)
#define rall(x) rbegin(x), rend(x)
template<class T> bool chmax(T& a, const T& b) { if (a < b) { a = b; return true; } else return false; }
template<class T> bool chmin(T& a, const T& b) { if (b < a) { a = b; return true; } else return false; }
using ll = long long;
using P = pair<int,int>;
using VI = vector<int>;
using VVI = vector<VI>;
using VL = vector<ll>;
using VVL = vector<VL>;

} int main() {
  ios::sync_with_stdio(false);
  cin.tie(0);
  int n;
  cin >> n;
  string s, t;
  cin >> s >> t;
  if (s.front() != t.front() || s.back() != t.back()) {
    cout << -1 << '\n';
    return 0;
  }
  VI ds, dt;
  rep(_, 2) {
    rep(i, n - 1) if ((s[i] != s[i + 1]) ^ (i & 1)) ds.emplace_back(i);
    swap(s, t);
    swap(ds, dt);
  }
  if (ds.size() != dt.size()) {
    cout << -1 << '\n';
    return 0;
  }
  int sz = ds.size();
  ll ans = 0;
  rep(i, sz) ans += abs(ds[i] - dt[i]);
  cout << ans << '\n';
}