a1,a2,a3,a4=input("４つの値を入力してください").split()
b1=int(a1)
b2=int(a2)
b3=int(a3)
b4=int(a4)
b=[b1,b2,b3,b4]
b.sort()
if((b[0]+1==b[1])and(b[1]+1==b[2])and(b[2]+1==b[3])):
    print("yes")
else:
    print("no")