MOD = 998244353
mr6 = pow(6, -1, MOD)

def calc(x):
    ret = (x * (x + 1)) % MOD
    ret *= (2 * x + 1)
    ret %= MOD
    ret *= mr6
    ret %= MOD
    return ret


M, N = map(int, input().split())
X = list(map(int, input().split())) + [M + 1]
pre = 0
ans = 0
for x in X:
    ans += calc(x - pre - 1)
    ans %= MOD
    pre = x
print(ans)