#include<bits/stdc++.h>
#define rep(i,n) for(int i=0;i<(int)n;i++)
#define all(c) (c).begin(),(c).end()
#define pb push_back
#define dbg(...) do{cerr<<__LINE__<<": ";dbgprint(#__VA_ARGS__, __VA_ARGS__);}while(0);

using namespace std;

namespace std{template<class S,class T>struct hash<pair<S,T>>{size_t operator()(const pair<S,T>&p)const{return ((size_t)1e9+7)*hash<S>()(p.first)+hash<T>()(p.second);}};template<class T>struct hash<vector<T>>{size_t operator()(const vector<T> &v)const{size_t h=0;for(auto i : v)h=h*((size_t)1e9+7)+hash<T>()(i)+1;return h;}};}
template<class T>ostream& operator<<(ostream &os, const vector<T> &v){os<<"[ ";rep(i,v.size())os<<v[i]<<(i==v.size()-1?" ]":", ");return os;}template<class T>ostream& operator<<(ostream &os,const set<T> &v){os<<"{ "; for(const auto &i:v)os<<i<<", ";return os<<"}";}
template<class T,class U>ostream& operator<<(ostream &os,const map<T,U> &v){os<<"{";for(const auto &i:v)os<<" "<<i.first<<": "<<i.second<<",";return os<<"}";}template<class T,class U>ostream& operator<<(ostream &os,const pair<T,U> &p){return os<<"("<<p.first<<", "<<p.second<<")";}
void dbgprint(const string &fmt){cerr<<endl;}template<class H,class... T>void dbgprint(const string &fmt,const H &h,const T&... r){cerr<<fmt.substr(0,fmt.find(","))<<"= "<<h<<" ";dbgprint(fmt.substr(fmt.find(",")+1),r...);}
typedef long long ll;typedef vector<int> vi;typedef pair<int,int> pi;const int inf = (int)1e9;const double INF = 1e12, EPS = 1e-9;

template<class T>struct SegTree{
	T *dat, *lazy;
	int n;
	SegTree(int size = 1000000){
		for(n = 1; n < size; n *= 2);
		dat = new T[2 * n - 1];
		lazy= new T[2 * n - 1];
		init();
	}
	~SegTree(){ delete [] dat; delete [] lazy; }
	
	void init(){
		rep(i, 2 * n - 1) dat[i] = 0;
		rep(i, 2 * n - 1) lazy[i]= 0;
	}
	void add(int a, int b, ll v, int k, int l, int r){
		if(r <= a || b <= l) return;
		if(a <= l && r <= b){ lazy[k] += v; return; }
		if(lazy[k]){
			lazy[k * 2 + 1] += lazy[k];
			lazy[k * 2 + 2] += lazy[k]; lazy[k] = 0;
		}
		add(a, b, v, k * 2 + 1, l, (l + r) / 2);
		add(a, b, v, k * 2 + 2, (l + r) / 2, r);
		dat[k] = min(dat[k * 2 + 1] + lazy[k * 2 + 1], dat[k * 2 + 2] + lazy[k * 2 + 2]);
	}
	T query(int a, int b, int k, int l, int r){
		if(r <= a || b <= l) return inf;
		if(a <= l && r <= b) return dat[k] + lazy[k];
		if(lazy[k]){
			lazy[k * 2 + 1] += lazy[k];
			lazy[k * 2 + 2] += lazy[k]; lazy[k] = 0;
		}
		T vl = query(a, b, k * 2 + 1, l, (l + r) / 2);
		T vr = query(a, b, k * 2 + 2, (l + r) / 2, r);
		return min(vl, vr);
	}
	T query(int a, int b){ return query(a, b, 0, 0, n); }
	void add(int a, int b, ll v){ add(a, b, v, 0, 0, n); }
};
int main(){
	cin.tie(0); cin.sync_with_stdio(0);
	
	int n, len; cin >> n >> len;
	string s, t; cin >> s >> t;
	
	vi ans(n + 1);
	SegTree<int> seg(n + 2);
	rep(i, n){
		int best = inf;
		if(s[i] == t[i]) best = ans[i];
		if(i && t[i - 1] != t[i]) seg.add(0, i, 1);
		best = min(best, (seg.query(max(0, i + 1 - len), i + 1) + 1) / 2 + 1);
		
		seg.add(i + 1, i + 2, 2 * best);
		ans[i + 1] = best;
	}
	cout << ans[n] << endl;
	return 0;
}