<s>[INST] Combination of problem having identical objects without using generating function approach 
A person goes in for an examination in which there are $4$ papers with a maximum of $m$ marks from each paper. The number of ways in which one can get $2m$ marks is

My Attempt
Number of ways of partitioning $n$ identical objects into $r$ distinct groups, if empty groups are allowed is:${}^{n+r-1}C_{r-1}$.
Here, $n=2m$ and $r=4$
Req. number of ways = ${}^{n+r-1}C_{r-1}-$[#{more than m marks for paper 1}+#{more than m marks for paper 2}+#{more than m marks for paper 3}+#{more than m marks for paper 4}]=$${}^{2m+4-1}C_{4-1}-[{}^{m+4-1}C_{4-1}+{}^{m+4-1}C_{4-1}+{}^{m+4-1}C_{4-1}+{}^{m+4-1}C_{4-1}]={}^{2m+4-1}C_{4-1}-4.{}^{m+4-1}C_{4-1}\\
={}^{2m+3}C_{3}-4.{}^{m+3}C_{3}=\frac{(2m+3)(2m+2)(2m+1)}{3.2}-4\frac{(m+3)(m+2)(m+1)}{3.2}\\
=\frac{m+1}{3}.[4m^2+8m+3-2m^2-10m-12]=\frac{(m+1)(2m^2-2m-9)}{3}
$$
But my reference gives the solution $\dfrac{(m+1)(2m^2+4m+3)}{3}$, so what is going wrong here ?
Note: I am trying to solve this problem without using the generating function approach
 [/INST] $\#$ of ways $n$ identical objects can be divided into $r$ distinct groups if empty groups are allowed is:${}^{n+r-1}C_{r-1}$
Req. # of ways=$${}^{2m+4-1}C_{4-1}-\Big[\text{#{> m for paper 1}+#{> m for paper 2}+#{> m for paper 3}+#{> m for paper 4}}\Big]\\+\Big[\text{#{strictly m for paper 1}+#{strictly m for paper 2}+#{strictly m for paper 3}+#{strictly m for paper 4}}\Big]\\
={}^{2m+4-1}C_{4-1}-4\Big[{}^{m+4-1}C_{4-1}\Big]+4\Big[{}^{m+3-1}C_{3-1}\Big]\\
={}^{2m+3}C_{3}-4\Big[{}^{m+3}C_{3}\Big]+4\Big[{}^{m+2}C_{2}\Big]\\
=\frac{(2m+3)(2m+2)(2m+1)}{3.2}-4.\frac{(m+3)(m+2)(m+1)}{3.2}+4.\frac{(m+2)(m+1)}{2}\\
=\frac{m+1}{3}.[4m^2+8m+3-2m^2-10m-12+6m+12]=\frac{m+1}{3}.[2m^2+4m+3]
$$
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