#include <cstdio>

#include <cstdlib>

#include <cstring>

#include <cmath>

#include <algorithm>

#include <assert.h>

#define IOS ios::sync_with_stdio(false)

using namespace std;

#define inf (0x3f3f3f3f)

typedef long long int LL;

#include <iostream>

#include <sstream>

#include <vector>

#include <set>

#include <map>

#include <queue>

#include <string>

void pr(int k, char ch) {

    for (int i = ; i <= k; ++i) {

        printf("%c", ch);

    }

}

void work() {

    int n, k, a, b;

    scanf("%d%d%d%d", &n, &k, &a, &b);

    if ((LL)k * (min(a, b) + ) < max(a, b)) {

        cout << "NO" << endl;

        return;

    }

    int to = ;

    int has = ;

    if (a > b) {

        while (to < n) {

            if (a > b && has < k) {

                printf("G");

                has++;

                a--;

                to++;

            } else {

                printf("B");

                b--;

                has = ;

                to++;

            }

        }

    } else {

        while (to < n) {

            if (b > a && has < k) {

                printf("B");

                has++;

                to++;

                b--;

            } else {

                printf("G");

                has = ;

                to++;

                a--;

            }

        }

    }

}

int main() {

#ifdef local

    freopen("data.txt", "r", stdin);

//    freopen("data.txt", "w", stdout);

#endif

    work();

    return ;

}