// STOOOOOOOOOOOOOOOOOOOOOOOOO hzt CCCCCCCCCCCCCCCCCCCCCCCORZ
#include <algorithm>
#include <cassert>
#include <cstring>
#include <iostream>
#include <numeric>
#include <vector>

using namespace std;
using LL = long long;
using PII = pair<int, int>;
constexpr int kN = 2024, kP = 1e9 + 7;
LL P(LL a, int b) {
  LL ret = 1;
  for (; b > 0; b /= 2) {
    if (b % 2 == 1)
      ret = ret * a % kP;
    a = a * a % kP;
  }
  return ret;
}

int n, m, a, b;
string s;
// int f[kN][kN][2];
int f[kN][2], g[kN][2];
int main() {
  cin.tie(0)->sync_with_stdio(0);
  cin >> n >> m >> a >> b;
  cin >> s, s = '#' + s;
  f[1][0] = (s[1] == 'B' || s[1] == '?');
  f[1][1] = (s[1] == 'G' || s[1] == '?');
  for (int i = 2; i <= n; i++) {
    memcpy(g, f, sizeof(f));
    memset(f, 0, sizeof(f));
    for (int cnt = 1; cnt <= i; cnt++) {
      if (s[i] == 'B' || s[i] == '?')
        f[cnt][0] = (g[cnt - 1][1] + g[cnt][0]) % kP, assert(cnt <= n && cnt - 1 >= 0);
      if (s[i] == 'G' || s[i] == '?')
        f[cnt][1] = (g[cnt][1] + g[cnt - 1][0]) % kP, assert(cnt <= n && cnt - 1 >= 0);
    }
  }
  LL ans = 0;
  for (int cnt = 1; cnt <= n; cnt++) {
    bool t0 = cnt / 2ll * a - (cnt - 1ll) / 2 * b >= m;
    bool t1 = (cnt - 1ll) / 2 * a - cnt / 2ll * b >= m;
    ans = (ans + t0 * f[cnt][0] + t1 * f[cnt][1]) % kP;
  }
  int q = count(s.begin(), s.end(), '?');
  cout << 1ll * ans * P((kP + 1) / 2, q) % kP << '\n';
  return 0;
}

该程序在运行第38行与第40行时，会出现 Runtime Error. Received signal 11: Segmentation fault with invalid memory reference.，关闭 O2 变为 WA，但这不重要，我很好奇这两句话怎么就越界了。我 assert 都成功了。