Let  ${\mathrm{x}}_1,{\mathrm{x}}_2,{\mathrm{x}}_3,....{\mathrm{x}}_{\mathrm{n}}$ be  $\mathrm{n}$ observations such that 
$\sum {\mathrm{x}}_{\mathrm{i}}^2=400$  and $\sum {\mathrm{x}}_{\mathrm{i}}=80$ , then $\mathrm{n}\ge \mathrm{k}$  then  $\mathrm{k}+2$ is