$$ n \geq 0 ,m\geq 1\\ \prod_{i=0}^{m}a_{n+i}-\prod_{i=0}^{m}a_{i}= \sum_{k=0}^{n-1}(a_{k+m+1}-a_{k})\prod_{i=1}^{m}a_{k+i} $$
$$ \sum_{k=0}^{n-1}(a_{k+m+1}-a_{k})\prod_{i=1}^{m}a_{k+i}=\sum_{k=0}^{n-1}a_{k+m+1}\prod_{i=1}^{m}a_{k+i}-\sum_{k=0}^{n-1}a_{k}\prod_{i=1}^{m}a_{k+i} \\= \sum_{k=1}^{n}a_{k+m}\prod_{i=1}^{m}a_{k+i-1}-\sum_{k=0}^{n-1}a_{k}\prod_{i=1}^{m}a_{k+i} \\=\sum_{k=1}^{n}a_{k+m}\prod_{i=0}^{m-1}a_{k+i}-\sum_{k=0}^{n-1}a_{k}\prod_{i=1}^{m}a_{k+i}\\=\sum_{k=1}^{n}\prod_{i=0}^{m}a_{k+i}-\sum_{k=0}^{n-1}\prod_{i=0}^{m}a_{k+i}\\=\prod_{i=0}^{m}a_{n+i}-\prod_{i=0}^{m}a_{i} $$