import sys
sys.setrecursionlimit(1000000000)
import math
import heapq
import time
from collections import deque
from collections import defaultdict
mod = 10**9+7
def solve(n,k,a,b):
if k*(min(a,b)+1)<max(a,b):
return "NO"
res = ""
turn = 0
while max(a,b)-min(a,b)>=k:
if turn%2==0:
if max(a,b)==a:
res += "G"*k
a-=k
else:
res += "B"*k
b-=k
else:
if min(a,b)==a:
res += "G"
a -=1
else:
res += "B"
b-=1
turn +=1
if len(res)==0:
if max(a,b) == a:
res += "G"
else:
res += "B"
while min(a,b)>0:
if res[-1]=="G":
res+= "B"
b-=1
else:
res += "G"
a-=1
while len(res)<n:
if max(a,b)==a:
res += "G"
else:
res += "B"
return res
n,k,a,b = map(int,input().split())
res2 = solve(n,k,a,b)
print(res2)
#include <stdio.h>
#include <stdlib.h>
#include <string.h>
#include <math.h>
#include <iostream>
#include <algorithm>
#include <vector>
#include <string>
#include <queue>
#include <map>
#include <set>
#include <stack>
#include <utility>
#include <complex>
#define ff first
#define ss second
using namespace std;
typedef long long LL;
typedef unsigned long long ULL;
const int INF=1e9+250;
LL mod=1e9+7;
const int N=100005;
char ans[N];
int main() {
#ifndef ONLINE_JUDGE
freopen("in.txt","r",stdin);
#endif
int n,m,a,b;
cin>>n>>m>>a>>b;
int pos=0;
int ta=0,tb=0;
while(a>0||b>0) {
if(a<b) {
if(tb<m) {
ans[pos]='B';
tb++;
ta=0;
b--;
} else {
if(a>0&&ta<m) {
ta++;
tb=0;
ans[pos]='G';
a--;
} else {
puts("NO");
return 0;
}
}
} else if(a>b) {
if(ta<m) {
ta++;
tb=0;
ans[pos]='G';
a--;
} else {
if(b>0&&tb<m) {
tb++;
ta=0;
ans[pos]='B';
b--;
} else {
puts("NO");
return 0;
}
}
} else {
if(ta<m) {
ta++;
tb=0;
ans[pos]='G';
a--;
} else if(tb<m) {
tb++;
ta=0;
ans[pos]='B';
b--;
} else {
puts("NO");
return 0;
}
}
pos++;
}
puts(ans);
return 0;
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