This is LaTeX code:

[latex]
 
N + 1 = \sum\limits_{i = 1}^k {\left( \begin{array}{c}
 x_i \\ 
 i \\ 
 \end{array} \right)} + 1 = \sum\limits_{i = 2}^k {\left( \begin{array}{c}
 x_i \\ 
 i \\ 
 \end{array} \right)} + \left( \begin{array}{c}
 x_1 \\ 
 1 \\ 
 \end{array} \right) + \left( \begin{array}{c}
 x_1 \\ 
 0 \\ 
 \end{array} \right) = ... = \left( \begin{array}{c}
 x_k + 1 \\ 
 k \\ 
 \end{array} \right)
 
[/latex]