Let $a_{k-1}+a_k=b_k^2$ $(b_k-1)^2<2a_{k-1}<a_{k-1}+a_k=b_k^2$ $a_k+a_{k+1}>2a_k>b_k^2$ and $(b_k+1)^2-2a_k=(b_k+1)^2-2(b_k^2-a_{k-1})=2b_k+1+2a_{k-1}-b_k^2>2b_k+1+(b_k-1)^2-b_k^2=2$ so $b_{k+1}=b_k+1=b_1+k$ $a_{k}=b_{k}^2-a_{k-1},a_{k+1}=(b_{k}+1)^2-a_k=2b_k+1+a_{k-1},a_{k+2}=(b_k+2)^2-a_{k+1}=b_k^2+2b_k+3-a_{k-1}$ $a_{k+2}-a_{k+1}=b_k^2+2-2a_{k-1}=a_k-a_{k-1}+2$ So every number in form $2k+a_1-a_0$ or $2k+a_2-a_1$ can be written in form $a_k-a_l$ And all numbers that can not be written in such form are $a_1-a_0-2k$ or $a_2-a_1-2k$ for $k>0$ There are $[\frac{a_1-a_0-1}{2}]+[\frac{a_2-a_1-1}{2}]=\frac{a_2-a_0-3}{2}=b_1-1=[\sqrt{2a_0}]$ such numbers