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

ans = 0
mod = 998244353
for i in range(len(X)-1):
    n = X[i+1] - X[i]
    ans += n * (n-1) * (2*n-1) // 6
    ans %= mod
print(ans)