#include<bits/stdc++.h>

#define lowbit(x) x&(-x)

#define LL long long

#define N 200005

#define M 1000005

#define mod 2147483648LL

#define inf 0x7ffffffff

using namespace std;

inline int ra()

{

    int x=,f=; char ch=getchar();

    while (ch<'' || ch>''){if (ch=='-') f=-; ch=getchar();}

    while (ch>='' && ch<=''){x=x*+ch-''; ch=getchar();}

    return x*f;

}

bool ans[N];

int cnt;

int main()

{

    int n=ra(),k=ra(),a=ra(),b=ra();

    bool last=;

    int nn=n;

    if (a>b) last=;

    int tot=;

    while (n--)

    {

        if (tot<k)

        {

            if (a>b) tot++,a--,ans[++cnt]=,last=;

                else tot++,b--,ans[++cnt]=,last=;

            if (a==b) break;

        }

        else

        {

            if (last) ans[++cnt]=,last^=,b--;

                else ans[++cnt]=,last^=,a--;

            tot=;

        }

        if (a< || b<)

        {

            cout<<"NO";

            return ;

        }

    }

    for (int i=; i<=n; i++)

        if (last) ans[++cnt]=,last^=;

            else ans[++cnt]=,last^=;

    for (int i=; i<=nn; i++)

        if (ans[i]) cout<<"G";

            else cout<<"B";

    return ;

}

//原来死都做不出来，看起来是淡定啊。。。2333

//不知道怎么搞CSDN表示尴尬2333