STPAR.py

MAX = 1005
stack = [0] * MAX

while(1):
    n = int(raw_input())
    if n == 0:
        break

    order = map(int, raw_input().split())

    st_size = 0
    check = 1
    start = 1

    for i in range(0, n):
        while (st_size > 0 and stack[st_size - 1] == start):
            st_size -= 1
            start += 1

        if order[i] == start :
            start += 1
        elif st_size > 0 and stack[st_size - 1] < order[i]:
            check = 0
            break
        else:
            stack[st_size] = order[i]
            st_size += 1

    if check == 1:
        print "yes\n"
    else:
        print "no\n"