def is_arithmetic_sequence(n: int, seq: list[int]) -> str:
    diff = seq[1] - seq[0]
    for i in range(2, n):
        if seq[i] - seq[i-1] != diff:
            return "NO"
    return "YES"

n = int(input().strip())
seq = list(map(int, input().split()))
print(is_arithmetic_sequence(n, seq))