#include <bits/stdc++.h>
using namespace std;
const int MOD = (int)1e9 + 7;
template <typename T>
static T aabs(T x) {
  if (x < 0) return -x;
  return x;
}
template <typename T>
static void read(T &S) {
  char K[1000009];
  scanf("%s", &K);
  S = K;
}
template <typename T, typename U>
static void amax(T &x, U y) {
  if (x < y) x = y;
}
template <typename T, typename U>
static void amin(T &x, U y) {
  if (x > y) x = y;
}
template <typename T, typename U>
static void aswap(T &x, U &y) {
  T temp;
  temp = x;
  x = y;
  y = temp;
}
template <typename T, typename U>
static long long gcd(T a, U b) {
  long long r;
  while (b) {
    r = a % b;
    a = b;
    b = r;
  }
  return a;
}
template <typename T, typename U>
static long long lcm(T a, U b) {
  return a / gcd(a, b) * b;
}
template <typename T, typename U>
static long long fpow(T n, U k, int p = MOD) {
  long long r = 1;
  for (; k; k >>= 1) {
    if (k & 1) r = r * n % p;
    n = n * n % p;
  }
  return r;
}
const int N5 = 100009;
const int N6 = 1000009;
int main() {
  int ma, n, k, a, b;
  scanf("%d%d%d%d", &n, &k, &a, &b);
  ma = 0;
  if (a == b) {
    for (int i = (1); i <= (a); i++) printf("GB");
  } else {
    int x;
    x = min(a, b);
    a -= x;
    b -= x;
    if (a) {
      --a;
      if (a > (x + 1) * (k - 1)) {
        printf("NO\n");
        return 0;
      }
      for (int i = (1); i <= (x); i++) {
        int sl;
        sl = 1;
        if (a > 0) {
          sl = min(k - 1, a);
          ++sl;
          a -= k - 1;
          for (int j = 0; j < (sl); j++) printf("G");
        } else {
          printf("G");
        }
        printf("B");
      }
      if (a > 0) {
        for (int j = 0; j < (1 + a); j++) printf("G");
      } else
        printf("G");
    } else {
      --b;
      if (b > (x + 1) * (k - 1)) {
        printf("NO\n");
        return 0;
      }
      for (int i = (1); i <= (x); i++) {
        int sl;
        if (b > 0) {
          sl = min(k - 1, b);
          ++sl;
          b -= k - 1;
          for (int j = 0; j < (sl); j++) printf("B");
        } else
          printf("B");
        printf("G");
      }
      if (b > 0)
        for (int j = 0; j < (1 + b); j++) printf("B");
      else
        printf("B");
    }
  }
  printf("\n");
  return 0;
}
