max=input()
while max:
        seq=map(int, raw_input().split())
        tail=0
        appendix=[]
        for truck in seq:
                if truck==tail+1: tail=truck
                else: appendix.append(truck)
                while appendix:
                        if appendix[-1]==tail+1: tail=appendix.pop()
                        else: break
        if appendix: print 'no'
        else: print 'yes'
        max=input()