M, N = map(int, input().split())
X = [0] + list(map(int, input().split())) + [M + 1]
ans = 0
mod = 998244353
for i in range(len(X) - 1):
    d = X[i + 1] - X[i] - 1
    ans += d * (d + 1) * (2 * d + 1) // 6
    ans %= mod

print(ans)