m, n = map(int, input().split())
x = [0] + list(map(int, input().split())) + [m+1]
mod = 998244353

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