#include<bits/stdc++.h>
using namespace std;
#define int long long
#define pb push_back
#define pii pair<int,int>
#define mk make_pair
#define fr first
#define se second
const int Maxn=2e2+10;
const int Maxm=1e3+10;
const int N=5e5+10;
const int M=2e6+10;
const int mod=998244353;
inline int read(){
	int x=0,f=1;char ch=getchar();
	while(ch<'0'||ch>'9'){
		if(ch=='-')f=-1;ch=getchar();
	}
	while(ch>='0'&&ch<='9'){
		x=(x<<3)+(x<<1)+(ch^48);ch=getchar();
	}
	return x*f;
} 
inline void write(int x){
	if(x<0){
		x=-x;putchar('-');
	}
	if(x>9)write(x/10);
	putchar(x%10+'0');
}
inline int ksm(int a,int b){
	int ans=1;
	while(b){
		if(b&1)ans=ans*a%mod;b>>=1;a=a*a%mod;
	}
	return ans;
}
int n,m;
char a[M];
int sum[M];int f[M];
deque<int>pq;
signed main(){
//	freopen("sing.in","r",stdin);
//	freopen("sing.out","w",stdout);
	n=read(),m=read();
	for(int i=1;i<=n;i++){
		cin>>a[i];
		sum[i]=sum[i-1];
		if(a[i]=='H')sum[i]++;
		else sum[i]--;
	}
	pq.pb(0);
	for(int i=1;i<=n;i++){
		while(!pq.empty()&&i-pq.front()>m)pq.pop_front();
		f[i]=f[pq.front()]+(a[i]-a[pq.front()]>=0);
		while(!pq.empty()&&(f[pq.back()]>f[i]||(f[pq.back()]==f[i]&&sum[pq.front()]<sum[i])))pq.pop_back();
		pq.pb(i);
	}
	write(f[n]);
	return 0;
}