M, N = map(int, input().split())
X = list(map(int, input().split()))
MOD = 998244353

def inv(x):
    return pow(x, MOD-2, MOD)
def sm(x):
    res = x * (x+1) * (2*x+1)
    res *= inv(6)
    res %= MOD
    return res

L = []
prev = 0
for i in range(N):
    x = X[i]
    if x - prev - 1 > 0:
        L.append(x - prev - 1)
    prev = x
x = M+1
if x - prev - 1 > 0:
    L.append(x - prev - 1)
# print(L)

ans = 0
for item in L:
    ans += sm(item)
    ans %= MOD
print(ans)