def main():
    import sys
    input = sys.stdin.read().split()
    n = int(input[0])
    a = list(map(int, input[1:n+1]))
    
    if n == 1:
        print("Yes")
        return
    
    # Check if all consecutive differences are <=1 in absolute value
    for i in range(n-1):
        if abs(a[i+1] - a[i]) > 1:
            print("No")
            return
    
    # Check if it's a mountain array
    # Find the peak
    peak = -1
    for i in range(1, n):
        if a[i] > a[i-1]:
            continue
        else:
            peak = i-1
            break
    else:
        # All increasing
        peak = n-1
    
    # Check if all elements after peak are strictly decreasing
    for i in range(peak, n-1):
        if a[i] <= a[i+1]:
            print("No")
            return
    
    # Check if all elements before peak are strictly increasing
    for i in range(peak):
        if a[i] >= a[i+1]:
            print("No")
            return
    
    print("Yes")

if __name__ == "__main__":
    main()