#include <bits/stdc++.h>
using namespace std;

#define FOR(i, a, b) for(int i = (a); i < (b); i++)
#define RFOR(i, a, b) for(int i = (a) - 1; i >= (b); i--)
#define SZ(a) int(a.size())
#define ALL(a) a.begin(), a.end()
#define PB push_back
#define MP make_pair
#define F first
#define S second

typedef long long LL;
typedef vector<int> VI;
typedef pair<int, int> PII;
typedef double db;

int main()
{
	ios::sync_with_stdio(0); 
	cin.tie(0);
	int n, c, p, q;
	string s;
	cin >> n >> c >> p >> q >> s;
	VI a(n + 1), r(n + 1);
	vector<LL> f(n + 1);
	FOR(i, 0, n)
		a[i + 1] = a[i] + (s[i] == 'Y');
	FOR(i, 0, n + 1)
		f[i] = (LL)a[i] * q - (LL)i * p;
	multiset<LL> ms;
	int le = 0, ri = 0;
	FOR(i, 0, n)
	{
		while (r[le] != r[i])
		{
			auto it = ms.find(f[le]);
			if (le < ri)
			{
				assert(it != ms.end());
				ms.erase(it);
			}
			le++;
		}
		while (ri <= i - c + 1)
		{
			if (le <= ri)
				ms.insert(f[ri]);
			ri++;
		}
		r[i + 1] = r[i];
		if (!ms.empty() && *ms.begin() <= f[i + 1])
		{
			r[i + 1]++;
		}
	}
	cout << r[n] << "\n";
	return 0;
}