Friends problem

A class has 100 students .Let $a_i$ $\leq$i$\leq$100, denotes the number of friends the i – th student has in the class . For each 0 $\leq$ j $\leq$99 , let $c_j$ denote the number of students having atleast j friends . Show that $\sum_{i=1}^m a_i$ = $\sum_{i=0}^n c_i$ , where m = 100 and n = 99.

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