import sys
input = sys.stdin.readline

mod=998244353

M,N=map(int,input().split())
X=[0]+list(map(int,input().split()))+[M+1]

ANS=0

for i in range(1,len(X)):
    k=X[i]-X[i-1]

    ANS+=k*(k-1)*(2*k-1)//6
    ANS%=mod

print(ANS)