#include<bits/stdc++.h>
#define ll long long
using namespace std;
long long read(){
	long long x=0,f=1;char ch=getchar();
	while(!isdigit(ch))
	{if(ch=='-') f=-1;ch=getchar();}
	while(isdigit(ch)){x=x*10+ch-48;ch=getchar();}
	return x*f;
}
void write(long long x){
    if(x<0) putchar('-'),x=-x;
    if(x>9) write(x/10);
    putchar(x%10+'0');
}
const int N=3e5+10;
int n,k;
string s;
int a[N];
int f[N],q[N],tl,tr;
int main(){
	//freopen(".in","r",stdin);
	//freopen(".out","w",stdout);
	n=read();k=read();
    cin>>s;
    s=" "+s;
    for(int i=1;i<=n;i++){
        a[i]=a[i-1]+(s[i]=='H'?-1:1);
    }
    q[tl=tr=1]=0;
    for(int i=1;i<=n;i++){
        while(tl<=tr&&i-q[tl]>k)tl++;
        f[i]=f[q[tl]]+(a[i]-a[q[tl]]>=0);
        while(tl<=tr&&(f[q[tr]]>f[i]||(f[q[tr]]==f[i]&&a[q[tr]]<a[i])))tr--;
        q[++tr]=i;
    }
    write(f[n]);puts("");
	return 0;
}