#coding:utf-8

H,W,N,K=map(int,raw_input().split())

all=H*W

cnt=0
while True:
	for i in range(1,N+1):
		all-=i
		cnt+=1
		if all < 0:
			break
	
	break

if cnt==K:
	print 'YES'
else:
	print 'NO'