#include<bits/stdc++.h>
using namespace std;
using ll=long long;

bool DEB=0;

void naive(){

}
using vll=vector<ll>;
#define rep(i,n) for(int i=0;i<n;i++)

struct Fenwick_tree {
	ll n;
	vll bit;
	Fenwick_tree(ll num) :bit(num + 1, 0) { n = num; }

	void add(ll i, ll w) {
		for (ll x = i+1; x <= n; x += x & -x) {
			bit[x-1] += w;
		}
	}

	ll sum(ll i) {
		ll ret = 0;
		for (ll x = i+1; x > 0; x -= x & -x) {
			ret += bit[x-1];
		}
		return ret;
	}

	ll sum(ll L, ll R) {
		return sum(R) - sum(L);
	}


};
//順列Aの転倒数. 
ll TE(vll A) {
	ll res = 0;
	ll N = A.size();
	Fenwick_tree fw(N);
	rep(i, N) {
		res += i - fw.sum(A[i]);
		fw.add(A[i], 1);
	}
	return res;
}
//数列A,Bの転倒数
ll TTE(vll A,vll B){
    ll res=0;
    ll N=A.size();
    map<ll,vll> PB;
    rep(i,N){
        PB[B[i]].push_back(i);
    }
    vll CA(N);
    for(ll n=N-1;n>=0;n--){
        CA[n]=PB[A[n]].back();
        PB[A[n]].pop_back();
    }
    return TE(CA);
}

vector<ll> V(vector<int> D){
	int N=D.size();
	vector<ll> R(N);
	int cnt=0;
	for(int i=0;i<N;i++){
		if(D[i]==0){
			R[i]=cnt;
			cnt++;
		}
	}
	for(int i=0;i<N;i++){
		if(D[i]==1){
			R[i]=cnt;
			cnt++;
		}
	}
	return R;
}

void solve(){
	int N;
	cin>>N;
	string A,B;
	cin>>A>>B;
	if(A[0]!=B[0]||A[N-1]!=B[N-1]){
		cout<<-1<<"\n";
		return;
	}
	vector<int> D(N-1),E(N-1);
	ll c=0;
	for(int i=0;i<N-1;i++){
		D[i]=((A[i]-'A')^(A[i+1]-'A'));
	}
	for(int i=0;i<N-1;i++){
		E[i]=((B[i]-'A')^(B[i+1]-'A'));
	}

	for(int i=0;i<N-1;i++){
		if(i%2==1){
			D[i]=1-D[i];
			E[i]=1-E[i];
		}
		c+=D[i];
		c-=E[i];
	}
	
	if(c!=0){
		cout<<-1<<"\n";
		return;
	}
	cout<<TTE(V(D),V(E))<<"\n";
}

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

	int T=1;
	// cin>>T;
	while(T--)solve();
	
}