mod = 998244353

m, n = map(int, input().split())
x = [0] + list(map(int, input().split())) +[m + 1]

def f(n):
    return n * (n + 1) * (n * 2 + 1) // 6 

ans = 0
for i in range(len(x) - 1):
    a = x[i+1] - x[i] - 1
    ans += f(a)
    ans %= mod
    
print(ans)