m,n=map(int,input().split())
x=list(map(int,input().split()))
ans=0
x=[0]+x
mod=998244353
def f(k):
    return ((k*(k+1)*(2*k+1))//6)%mod
for i in range(1,n+1):
    ans+=f(x[i]-x[i-1]-1)
    ans%mod
ans+=f(m-x[-1])
print(ans%mod)