#include <bits/stdc++.h>
using namespace std;
int main() {
  long long int n, k, a, b, x1, x2, p, r, i, r1, flag2 = 0;
  char s[100009], t1, t2;
  cin >> n >> k >> a >> b;
  if (a >= b) {
    t1 = 'G';
    t2 = 'B';
    x1 = a;
    x2 = b;
  } else {
    t1 = 'B';
    t2 = 'G';
    x1 = b;
    x2 = a;
  }
  i = 1;
  r = 0;
  while (1) {
    if (i % 2 != 0) {
      p = x1 - x2;
      if (p <= 0) {
        p = 1;
      }
      if ((x1 == 0 && x2 != 0)) {
        flag2 = 1;
        break;
      } else if (x1 == 0 && x2 == 0) {
        break;
      }
      if (p <= k) {
        if (p == 0) {
          p = 1;
        }
        for (r1 = 1; r1 <= p; r1++) {
          s[r] = t1;
          r++;
        }
        x1 = x1 - p;
      } else if (p > k) {
        for (r1 = 1; r1 <= k; r1++) {
          s[r] = t1;
          r++;
        }
        x1 = x1 - k;
      }
    } else if (i % 2 == 0) {
      p = x2 - x1;
      if (p <= 0) {
        p = 1;
      }
      if ((x2 == 0 && x1 != 0)) {
        flag2 = 1;
        break;
      } else if (x1 == 0 && x2 == 0) {
        break;
      }
      if (p <= k) {
        for (r1 = 1; r1 <= p; r1++) {
          s[r] = t2;
          r++;
        }
        x2 = x2 - p;
      } else if (p > k) {
        for (r1 = 1; r1 <= k; r1++) {
          s[r] = t2;
          r++;
        }
        x2 = x2 - k;
      }
    }
    i++;
  }
  s[r] = '\0';
  if (flag2 == 0) {
    cout << s;
  } else
    cout << "NO";
}
