#include<bits/stdc++.h>
using namespace std;
constexpr int M=5e5+5;
int n,k,dp[M],cntB[M],cntW[M];
string s,t;int h0,h1,t0,t1,q0[M],q1[M];
int read(){
    int x=0;char ch=getchar();
    while (!isdigit(ch)) ch=getchar();
    while (isdigit(ch)) x=x*10+ch-48,ch=getchar();
    return x;
}
int main(){
    ios::sync_with_stdio(0); cin.tie(0); cout.tie(0);
    cin>>n>>k>>s>>t;s=' '+s;t=' '+t;
    for (int i=1;i<=n;i++)
        cntB[i]=cntB[i-1]+(t[i-1]!='B'&&t[i]=='B'),
        cntW[i]=cntW[i-1]+(t[i-1]!='W'&&t[i]=='W');
    for (int i=1;i<=n;i++){
        auto calc0=[&](auto j){return dp[j]-cntB[j]+(t[j+1]=='B')+1;};
        auto calc1=[&](auto j){return dp[j]-cntW[j]+(t[j+1]=='W')+1;};
        while (h0<=t0&&q0[h0]<i-k) h0++;
        while (h1<=t1&&q1[h1]<i-k) h1++;
        dp[i]=min({dp[i-1]+(s[i]!=t[i]),
                cntB[i]+calc0(q0[h0]),
                cntW[i]+calc1(q1[h1])});
        while (h0<=t0&&calc0(q0[t0])>=calc0(i)) t0--;
        while (h1<=t1&&calc1(q1[t1])>=calc1(i)) t1--;
        q0[++t0]=i; q1[++t1]=i;
    }
    printf("%d\n",dp[n]);
    return 0;
}