def inp() a=gets.chomp.split(" ").map(&:to_i)end def inpf() a=gets.chomp.split(" ").map(&:to_f)end def inps() a=gets.chomp.split(" ")end def copy(a) Marshal.load(Marshal.dump(a)) end def kaizyo(n)(n < 2)? 1 : (2..n).inject(:*) end def scount(a) b=na(a.max+1);a.each{|n|b[n]+=1};return b end def na(n,d=0) Array.new(n,d)end def na2(n,m,d=0) Array.new(n){Array.new(m,d)}end def na3(n,m,l,d=0) Array.new(n){Array.new(m){Array.new(l,d)}}end def bit(n) a.to_s(2).split("").map(&:to_i) end n=inp[0] a=inp t = 0 i = n a.reverse_each do |d| unless((d - t) == i) t+= 1 end i -= 1 end p t