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

mod=998244353
ans=0
before=0
for xi in x:
    if xi-before-1==0:
        before=xi
        continue
    n=xi-before-1
    # print(before, xi, n)
    ans+=n*(n+1)*(2*n+1)//6
    ans%=mod
    before=xi
if m-before!=0:
    n=m-before
    # print(before, m, n)
    ans+=n*(n+1)*(2*n+1)//6
    ans%=mod
print(ans)