require'prime'
m,n,*c=$<.read.split.map &:to_i
(d=[-m]*m<<r=0).map{k=d[m];c.map{|j|j>m||d[m-j]=[d[m-j],k+1].max};m.prime?&&k>0&&r+=k;m-=1}
p d.max+r