hi

n, a = int(input()), input().split()
a = [int(j) for j in a]

i = 1
f=0
for i in range(i, n):
if not (a[i – 1] < a[i]):
break
else:
print(“yes”)
f+=1

if f!=1:
for i in range(i,n):
if not (a[i-1] == a[i]):
break
else:
print(“yes”)
f+=1
if f!=1:
for i in range(i,n):
if not (a[i – 1] > a[i]):
print(“no”)
break
else:
print(“yes”)