import sys
input = lambda :sys.stdin.readline()[:-1]
ni = lambda :int(input())
na = lambda :list(map(int,input().split()))
yes = lambda :print("yes");Yes = lambda :print("Yes");YES = lambda : print("YES")
no = lambda :print("no");No = lambda :print("No");NO = lambda : print("NO")
#######################################################################
m, n = na()
mod = 998244353
x = [0] + na() + [m + 1]
s = 0

ans = 0

def f(s, h):# s^2 + (s + 1) ^ 2 + ... + (s + h - 1) ^ 2
    return g(s + h - 1) - g(s - 1)

def g(x):
    return x * (x + 1) * (2 * x + 1) // 6 % mod

for i in range(n + 1):
    h = x[i + 1] - x[i] - 1
    ans += g(h)
    ans %= mod

print(ans)