## https://yukicoder.me/problems/no/2940


MOD = 998244353

inv6 = pow(6, MOD - 2, MOD)

def sigma_n2(n):
    ans = (n * (n + 1) )% MOD
    ans *= (2 * n + 1) % MOD
    ans %= MOD
    ans *= inv6
    ans %= MOD
    return ans


def main():
    M, N = map(int, input().split())
    X = list(map(int, input().split()))

    X_ = [0] + X + [M + 1]
    answer = 0
    for i in range(len(X_) - 1):
        x0 = X_[i]
        x1 = X_[i + 1]
        if x1 - x0 > 1:
            n = x1 - x0 - 1

            a = sigma_n2(n)
            answer += a
            answer %= MOD
    print(answer)














if __name__ == "__main__":
    main()