#include <bits/stdc++.h>
#define endl '\n'

//#pragma GCC optimize ("O3")
//#pragma GCC target ("sse4")

#define SZ(x) ((int)x.size())
#define ALL(V) V.begin(), V.end()
#define L_B lower_bound
#define U_B upper_bound
#define pb push_back

using namespace std;
template<class T, class T2> inline int chkmax(T &x, const T2 &y) { return x < y ? x = y, 1 : 0; }
template<class T, class T2> inline int chkmin(T &x, const T2 &y) { return x > y ? x = y, 1 : 0; }
const int MAXN = (1 << 20);
const int OFFSET = (int)3e5 + 152;

int n, k;
string s;

void read()
{
	cin >> n >> k >> s;
}

int psum[MAXN];
int dp[MAXN];

int tr[4 * MAXN];
multiset<int> P[OFFSET * 2];

void init(int l, int r, int idx)
{
	if(l == r)
	{
		tr[idx] = (int)1e9;
		return;
	}

	int mid = (l + r) >> 1;
	init(l, mid, 2 * idx + 1);
	init(mid + 1, r, 2 * idx + 2);

	tr[idx] = min(tr[2 * idx + 1], tr[2 * idx + 2]);
}

void add(int pos, int val, int l, int r, int idx)
{
	if(l == r)
	{
		P[OFFSET + pos].insert(val);
		tr[idx] = *P[OFFSET + pos].begin();
		return;
	}

	int mid = (l + r) >> 1;
	if(mid < pos) add(pos, val, mid + 1, r, 2 * idx + 2);
	else add(pos, val, l, mid, 2 * idx + 1);

	tr[idx] = min(tr[2 * idx + 1], tr[2 * idx + 2]);
}

void rem(int pos, int val, int l, int r, int idx)
{
	if(l == r)
	{
		P[OFFSET + pos].erase(P[OFFSET + pos].find(val));
		tr[idx] = P[OFFSET + pos].empty() ? (int)1e9 : *P[OFFSET + pos].begin();
		return;
	}

	int mid = (l + r) >> 1;
	if(mid < pos) rem(pos, val, mid + 1, r, 2 * idx + 2);
	else rem(pos, val, l, mid, 2 * idx + 1);

	tr[idx] = min(tr[2 * idx + 1], tr[2 * idx + 2]);
}

int query(int ql, int qr, int l, int r, int idx)
{
	if(qr < l || r < ql) return (int)1e9;
	if(ql <= l && r <= qr)
		return tr[idx];

	int mid = (l + r) >> 1;
	return min(query(ql, qr, l, mid, 2 * idx + 1), query(ql, qr, mid + 1, r, 2 * idx + 2));
}

void solve()
{
	for(int i = 1; i <= n; i++)
		psum[i] = psum[i - 1] + (s[i - 1] == 'G' ? 1 : -1);

	int N = n + 42;
	init(-N, N, 0);
	add(0, 0, -N, N, 0);
	for(int i = 1; i <= n; i++)
	{
		dp[i] = query(-N, psum[i], -N, N, 0) + 1;
		chkmin(dp[i], query(psum[i] + 1, N, -N, N, 0));
		
		add(psum[i], dp[i], -N, N, 0);
		if(i >= k) 
			rem(psum[i - k], dp[i - k], -N, N, 0);
	}

	cout << dp[n] << endl;
}

int main()
{
	freopen("redistricting.in", "r", stdin);
	freopen("redistricting.out", "w", stdout);
	ios_base::sync_with_stdio(false);
	cin.tie(NULL);

	read();
	solve();
	return 0;
}