/*
	Luogu name: Symbolize
	Luogu uid: 672793
*/
#include<bits/stdc++.h>
#define int long long
#define pii pair<int,int>
#define x first
#define y second
#define rep1(i,l,r) for(register int i=l;i<=r;++i)
#define rep2(i,l,r) for(register int i=l;i>=r;--i)
#define rep3(i,x,y,z) for(register int i=x[y];~i;i=z[i])
#define rep4(i,x) for(auto i:x)
#define debug() puts("----------")
const int N=1e6+10;
const int inf=0x3f3f3f3f3f3f3f3f;
using namespace std;
int n,c,p,q,cnt[N],rk[N];
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<<1)+(x<<3)+(ch^48);
		ch=getchar();
	}
	return f*x;
}
signed main()
{
//	freopen(".in","r",stdin);
//	freopen(".out","w",stdout);
	n=read();
	c=read();
	p=read();
	q=read();
	int Min=inf;
	rep1(i,1,n)
	{
		char ch;
		cin>>ch;
		cnt[i]=cnt[i-1]+(ch=='Y');
		rk[i]+=rk[i-1];
		if(i-c+1>=1&&rk[i-c+1]==rk[i]) Min=min(Min,q*cnt[i-c]-p*(i-c));
		if(q*cnt[i]-p*i>=Min)
		{
			++rk[i+1];
			Min=inf;
		}
	}
	rk[n+1]+=rk[n];
	cout<<rk[n+1]<<endl;
	return 0;
}
/*
(cnt[i]-cnt[j])/(i-j)>p/q
q*cnt[i]-q*cnt[j]>p*i-p*j
q*cnt[i]-p*i>q*cnt[j]-p*j
*/