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

def inverse(n, d):
    return n * pow(d, -1, MOD) % MOD

def func(n):
    return inverse(n*(n+1)%MOD*(2*n+1)%MOD, 6)

MOD = 998244353
ans = 0
for i in range(N+1):
    n = X[i+1]-X[i]-1
    ans += func(n)
    ans %= MOD

print(ans)