0046. Sum of Products
|Input file name:||sumprod.in|
|Output file name:||sumprod.out|
|Time limit:||1 s|
|Memory limit:||64 megabytes|
Of course you know that there are Ckn ways to select k numbers from a set of n. Given a set of n integer numbers, Anton for each such combination calculates its total product and then sums all products.
For example, if the numbers are (2, 2, 3), k=2 and n=3 he will get 2⋅ 2+2⋅ 3+2⋅ 3=16.
Your task is to calculate the answer he will get.
The first line of the input file contains two integer numbers n and k (1≤ k≤ n≤ 200). The second one contains n non-negative integer numbers. These numbers will not be greater than 32000.
Output only one integer number – sum of all products by k.
|3 2 2 2 3||16|
Source: Petrozavodsk training camp, Summer 2002. Conclusive contest
Author: Andrew Lopatin, Nick Durov
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