#include <iostream>
#include <vector>
#include <string>
#include <map>
#include <set>
#include <queue>
#include <algorithm>
#include <cmath>
#include <iomanip>
#include <random>
#include <stdio.h>
#include <fstream>
#include <functional>
#include <cassert>
#include <unordered_map>
#include <bitset>
#include <atcoder/all>

using namespace std;
using namespace atcoder;



#define rep(i,n) for (int i=0;i<n;i+=1)
#define rrep(i,n) for (int i=n-1;i>-1;i--)
#define pb push_back
#define all(x) (x).begin(), (x).end()

template<class T>
using vec = vector<T>;
template<class T>
using vvec = vec<vec<T>>;
template<class T>
using vvvec = vec<vvec<T>>;
using ll = long long;
using pii = pair<int,int>;
using pll = pair<ll,ll>;




template<class T>
bool chmin(T &a, T b){
  if (a>b){
    a = b;
    return true;
  }
  return false;
}

template<class T>
bool chmax(T &a, T b){
  if (a<b){
    a = b;
    return true;
  }
  return false;
}

template<class T>
T sum(vec<T> x){
  T res=0;
  for (auto e:x){
    res += e;
  }
  return res;
}

template<class T>
void printv(vec<T> x){
  for (auto e:x){
    cout<<e<<" ";
  }
  cout<<endl;
}

template<class T>
ostream& operator<<(ostream& os, const vec<T>& A){
  os << "{";
  rep(i,A.size()){
    os << A[i];
    if (i!=A.size()-1){
      os << ' ';
    }
  }
  os<<"}";
  return os;
}

template<class T,class U>
ostream& operator<<(ostream& os, const pair<T,U>& A){
  os << "(" << A.first <<", " << A.second << ")";
  return os;
}

template<class T>
ostream& operator<<(ostream& os, const deque<T>& A){
  os << "deque({";
  rep(i,A.size()){
    os << A[i];
    if (i!=A.size()-1){
      os << ' ';
    }
  }
  os<<"})";
  return os;
}

int pow(int a, int b, int mod){
  int res = 1;
  while (b){
    if (b&1){
      res = res * a % mod;
    }
    a = a * a % mod;
    b >>= 1;
  }
  return res;
}

using mint = modint998244353;

const mint i10 = mint(10).inv();

int main(){
  ios::sync_with_stdio(false);
  std::cin.tie(nullptr);

  int n;
  cin>>n;
  vec<int> A(n);
  rep(i,n){
    char s;
    cin>>s;
    A[i] = abs(s-'A'-(i/2)) % 2;
  }
  vec<int> B(n);
  rep(i,n){
    char s;
    cin>>s;
    B[i] = abs(s-'A'-(i/2)) % 2;
  }

  //cout << A << B << endl;

  if (A[0]!=B[0]){
    cout<< -1 << endl;
    return 0;
  }

  vec<int> RA;
  int c = A[0], k = 1;
  for (int i=1;i<n;i++){
    if (c==A[i]){
      k++;
    }
    else{
      RA.pb(k);
      c = A[i], k = 1;
    }
  }
  RA.pb(k);

  vec<int> RB;
  c = B[0], k = 1;
  for (int i=1;i<n;i++){
    if (c==B[i]){
      k++;
    }
    else{
      RB.pb(k);
      c = B[i], k = 1;
    }
  }
  RB.pb(k);

  if (RA.size()!=RB.size()){
    cout << -1 << endl;
    return 0;
  }

  //cout << RA << RB << endl;

  ll res = 0;
  vec<int> tmp_minus;
  vec<int> tmp_plus;
  rep(i,RA.size()){
    if (RA[i] > RB[i]){
      rep(_,RA[i]-RB[i]){
        tmp_minus.pb(i);
      }
    }
    else{
      rep(_,RB[i]-RA[i]){
        tmp_plus.pb(i);
      }
    }
  }

  

  rep(i,tmp_minus.size()){
    res += (ll)abs(tmp_minus[i]-tmp_plus[i]);
  }

  

  cout << res << endl;

  

  
  

  
}