#include <bits/stdc++.h>

#define FOR(i, x, n) for(int i = x; i < n; i++)
#define F0R(i, n) FOR(i, 0, n)

#define IOS ios_base::sync_with_stdio(0); cin.tie(0);

using namespace std;

typedef long long LL;

const int N = 5e5;
const LL INF = 1e18;

LL n, c, p, q;
LL have[N], rnk[N], dp[N];
string inp;

int main() {
    IOS;

    cin >> n >> c >> p >> q;
    cin >> inp;

    LL mx = -INF, large = 0;
    LL zero = 0;
    F0R(i, n) {
        if (i) dp[i] = dp[i - 1];

        zero += p;
        if (inp[i] == 'Y') zero -= q;
        have[i] = zero - p + (inp[i] == 'Y' ? q : 0);

        if (i - c + 1 >= 0 && (i - c >= 0 ? dp[i - c] : 0) == large) {
            mx = max(mx, have[i - c + 1]);
        }

        if (mx >= zero) {
            dp[i] = ++large;
            mx = -INF;
        }
    }

    cout << dp[n - 1] << endl;
    return 0;
}