m,n=map(int,input().split())
x=list(map(int,input().split()))

x=[0]+x[:]+[m+1]

mod=998244353
ans=0
for i in range(len(x)-1):
    N=x[i+1]-x[i]-1
    ans+=N*(N+1)*(2*N+1)//6

    ans%=mod

print(ans)