M, _ = map(int, input().split())
X = list(map(int, input().split()))
X = [0] + X + [M + 1]

ret = 0
for a, b in zip(X, X[1:]):
    d = b - a - 1
    ret += d * (d + 1) * (2 * d + 1) // 6

print(ret % 998244353)