4
90 91 90 91
 4
54 54 54 55
 10
55 39 93 42 97 40 36 38 11 97
 3
90 39 98
 5
21 57 40 94 17
 8
46 4 30 85 52 6 84 13
 6
47 100 96 2 96 43
 4
61 28 3 81
 5
14 91 91 91 84
 3
22 23 24
 1
13
 9
13 80 13 38 98 85 11 73 74
 10
95 51 52 8 44 39 77 17 96 88
 3
40 40 40
 4
16 17 17 18
 3
68 67 67
 4
83 100 81 75
 9
71 71 70 70 71 70 69 70 71
 10
9 9 9 10 10 9 8 8 9 9
 6
87 86 86 86 85 86
 5
64 63 63 62 61
 3
76 9 12
 4
61 60 60 60
 7
23 37 39 8 72 31 85
 10
67 67 67 67 67 67 67 67 67 67
 4
92 18 29 93
 4
14 15 15 16
 10
79 84 22 38 23 22 33 42 13 96
 7
20 21 95 73 49 98 53
 10
28 61 23 73 61 33 45 55 18 43
 9
15 14 14 13 13 12 13 13 14
 5
46 46 47 46 45
 7
35 91 87 78 17 71 63
 3
69 26 73
 10
16 17 16 15 14 15 16 17 16 15
 9
95 7 97 61 90 7 30 65 39
 10
2 83 18 65 58 95 37 51 86 47
 6
59 59 60 60 59 58

YES YES NO NO NO NO NO NO NO YES YES NO NO YES YES YES NO YES YES YES YES NO YES NO YES NO YES NO NO NO YES YES NO NO YES NO NO YES
n = int(input())
a = list(map(int, input().split()))
t = sorted(a)[:][::-1]
kek = []
for i in range(n):
    for j in range(len(a) - 1):
        if abs(a[j] - a[j + 1]) >= 2:
            if a[j] > a[j + 1]:
                a[j] -= 1
                a[j + 1] += 1
            else:
                a[j] += 1
                a[j + 1] -= 1
    mx = -1
    ind = -1
    for j in range(len(a)):
        if a[j] > mx:
            mx = a[j]
            ind = j
    del a[ind]
    #a.append(mx)
    #print(a)
    kek.append(mx)
    for j in range(len(a) - 1):
        if abs(a[j] - a[j + 1]) >= 2:
            if a[j] > a[j + 1]:
                a[j] -= 1
                a[j + 1] += 1
            else:
                a[j] += 1
                a[j + 1] -= 1
#print(kek)
for i in range(n):
    if kek[i] != t[i]:
        print('NO')
        exit(0)
print('YES')