#include <bits/stdc++.h>
#include <chrono> 

using namespace std;
using namespace std::chrono; 
 
// #pragma GCC target ("avx2")
// #pragma GCC optimization ("O3")
// #pragma GCC optimization ("unroll-loops")
// #pragma optimization_level 3
// #pragma GCC optimize("Ofast,no-stack-protector,unroll-loops,fast-math,O3")
// #pragma GCC target("sse,sse2,sse3,ssse3,sse4,popcnt,abm,mmx,avx,tune=native")

#define pb push_back
#define galen_colin {ios_base::sync_with_stdio(false);}
#define orz {cin.tie(NULL); cout.tie(NULL);}
#define fix(prec) {cout << setprecision(prec) << fixed;}
#define mp make_pair
#define f first
#define s second
#define all(v) v.begin(), v.end()

typedef long long ll;
typedef long double ld;
typedef unsigned long long ull;
typedef pair<int, int> pii;
typedef pair<ll, ll> pll;
typedef vector<int> vi;
typedef vector<ll> vl;
typedef vector<pii> vpi;
typedef vector<pll> vpl;
typedef vector<vi> vvi;
typedef vector<vl> vvl;

template<typename A> ostream& operator<<(ostream &cout, vector<A> const &v);
template<typename A, typename B> ostream& operator<<(ostream &cout, pair<A, B> const &p) { return cout << "(" << p.f << ", " << p.s << ")"; }
template<typename A> ostream& operator<<(ostream &cout, vector<A> const &v) {
	cout << ""; for(int i = 0; i < v.size(); i++) {if (i) cout << " "; cout << v[i];} return cout << "\n";
}
template<typename A, typename B> istream& operator>>(istream& cin, pair<A, B> &p) {
	cin >> p.first;
	return cin >> p.second;
}
template<typename A> istream& operator>>(istream& cin, vector<A> &v) {
    for(auto &x:v)cin>>x;
    return cin;
}

ll min(ll a, int b){return min(a, (ll) b);}
ll min(int a, ll b){return min(b, (ll) a);}
ll max(ll a, int b){return max(a, (ll) b);}
ll max(int a, ll b){return max(b, (ll) a);}

void usaco(string filename) {
  // #pragma message("be careful, freopen may be wrong")
	freopen((filename + ".in").c_str(), "r", stdin);
	freopen((filename + ".out").c_str(), "w", stdout);
}
 
const ld pi = 3.14159265358979323846;
const ll mod = 1000000007;
// const ll mod = 998244353;
// ll mod;
const ll INF = 1e9+7;
 


ll n, m, k, q, l, r, x, y, z;

const ll sz = 1e6 + 1;
ll a[sz] = {0};
ll b[sz];
ll c[sz];
string s, t;
ll ans = 0;

void solve(int tc = 0) {

	cin>>n>>x;
	cin>>s;
	for(int i=0;i<n;i++){
		if(s[i] == 'x')x = max(x-1, 0);
		else x++;
	}
    cout<<x<<"\n";
}
 
int main() {
	#ifdef HACKX
		auto begin = std::chrono::high_resolution_clock::now();
	#endif
	
	galen_colin orz
	
	// usaco("cowland");
 
	int tc = 1;
	// cin >> tc;
	for (int t = 0; t < tc; t++) solve(t);
	
	#ifdef HACKX
		auto end = std::chrono::high_resolution_clock::now();
		cout << setprecision(4) << fixed;
		// cout << "Execution time: " << std::chrono::duration_cast<std::chrono::duration<double>>(end - begin).count() << " seconds" << endl;
	#endif
} 
#pragma GCC optimize ("Ofast")
#define _USE_MATH_DEFINES
#include <bits/stdc++.h>
#define FOR(i, a, b) for (int i=(a); i<(b); i++)
#define FORD(i, a, b) for (int i=(a); i>(b); i--)
#define PPC(x) __builtin_popcount(x)
#define MSB(x) (63 - __builtin_clzll(x))
#define SZ(x) ((int)(x).size())
#define HASK(S, x) (S.find(x) != S.end())
#define pb push_back
#define ALL(x) (x).begin(), (x).end()
#define ithBit(m, i) ((m) >> (i) & 1)
#define ft first
#define sd second
#define kw(a) ((a) * (a))
#ifdef DEBUG
#include "debug.h"
#else
#define dbg(...) 0
#endif
using namespace std;

template <typename T1, typename T2> inline void remin(T1& a, T2 b) { a = min(a, (T1)b);	}

const int maxN = 1 << 19;

char T[maxN], S[maxN];
stack <int> pos;

void solve()
{
	int n;
	scanf ("%d", &n);
	scanf ("%s%s", S, T);
	FORD(i, n-1, -1)
		if (S[i] == '1')
			pos.push(i);
			
	long long res = 0;
			
	FOR(i, 0, n)
	{
		if (!pos.empty() and pos.top() == i)
			pos.pop();
			
		if (S[i] != T[i])	
		{
			if (pos.empty())
			{
				res = -1ll;
				break;
			}
			
			int j = pos.top();
			pos.pop();
			S[j] = '0';
			res += j - i;
		}	
	}
	
	printf("%lld\n", res);
}
 
int main()
{		
	int t = 1;
	//scanf ("%d", &t);
	FOR(tid, 1, t+1)
	{
		//printf("Case #%d: ", tid);
		solve();
	}
	return 0;
}
