#include<bits/stdc++.h>
using namespace std;
#include <unordered_set>
#include <random>
//#define int long long
#define REP(i,m,n) for(int i=(m);i<(n);i++)
#define rep(i,n) REP(i,0,n)
#define pb push_back
#define all(a) a.begin(),a.end()
#define rall(c) (c).rbegin(),(c).rend()
#define mp make_pair
#define endl '\n'
//#define vec vector<ll>
//#define mat vector<vector<ll> >
#define fi first
#define se second
#define double long double
typedef long long ll;
typedef unsigned long long ull;
typedef pair<ll,ll> pll;
//typedef long double ld;
typedef complex<double> Complex;
const ll INF=1e9+7;
const ll MOD=998244353;
const ll inf=INF*INF;
const ll mod=MOD;
const ll inv2=499122177;
const ll MAX=200010;
const double PI=acos(-1.0);
typedef vector<vector<ll> > mat;
typedef vector<ll> vec;

ll dx[]={0,1,0,-1,-1,-1,1,1};
ll dy[]={1,0,-1,0,-1,1,-1,1};

ll rui(ll a,ll b,ll mod){
    ll res=1;
    ll x=a;
    while(b){
        if(b&1)res=res*x%mod;
        x=x*x%mod;
        b/=2;
    }
    return res;
}


void solve(){
    ll N;cin>>N;
    string s,t;cin>>s>>t;
    if(s[0]!=t[0]||s[N-1]!=t[N-1]){
        cout<<-1<<endl;
        return;
    }
    ll ans=0;
    vector<ll>a(N),b(N);
    rep(i,N){
        if(s[i]=='B')a[i]=1;
        if(t[i]=='B')b[i]=1;
    }
    REP(i,1,N-1){
        if(a[i]!=b[i]){
            if(a[i-1]!=a[i+1]){
                cout<<-1<<endl;
                return;
            }else{
                a[i]=1-a[i];
                ans++;
            }
        }
    }
    cout<<ans<<endl;
}

signed main(){
    cin.tie(0);
    ios::sync_with_stdio(false);
    solve();
}