D. MaratonIME in the golden moment

Codeforces

IDCF10137D

Time2000ms

Memory256MB

Difficulty—

English · Original

Formal · Original

It is a common knowledge the importance in practicing a physical activity, mainly when you take part in ICPC competitions. Keeping this in mind, Giovana Delfino invited her friends from MaratonIME to go rowing. The initial total ability of the boat is 0. Despite the equal enthusiasm of all members, each friend i has a physical ability h(i). Dear teacher Gabi knows that whenever two friends x and y are together in the boat, the boat's total ability is increased by h(x)·h(y). Giovana invited n friends, but rowing is not always possible when you have programming assignments and exams of the great professor Arnaldo Mandel ahead, so sometimes not everyone shows up. However, the teacher Gabi is very optimistic and hopes that, for the final semester competition, all n friends will attend in order to increase MaratonIME odds. Help Gabi find the total ability of the boat in this possible golden moment, assuming all friends show up. The first line contains one integer n (1 ≤ n ≤ 105) - the number of friends invited to row. The second line contains n integers h1, h2, ..., hn (1 ≤ hi ≤ 3·104) - the abilities of each member. Print one integer - the boat's total ability in the golden moment. In the first example, in the golden moment there will be two members on the boat, then the ability there will be 5 × 10 = 50. ## Input The first line contains one integer n (1 ≤ n ≤ 105) - the number of friends invited to row.The second line contains n integers h1, h2, ..., hn (1 ≤ hi ≤ 3·104) - the abilities of each member. ## Output Print one integer - the boat's total ability in the golden moment. [samples] ## Note In the first example, in the golden moment there will be two members on the boat, then the ability there will be 5 × 10 = 50.

**Definitions** Let $ n \in \mathbb{Z}^+ $ be the number of friends. Let $ h = (h_1, h_2, \dots, h_n) $ be a sequence of positive integers representing the physical abilities of the friends. The initial boat ability is $ 0 $. **Constraints** 1. $ 1 \leq n \leq 10^5 $ 2. $ 1 \leq h_i \leq 3 \cdot 10^4 $ for all $ i \in \{1, \dots, n\} $ **Objective** Compute the total boat ability when all $ n $ friends are present, where the ability increases by $ h_i \cdot h_j $ for every unordered pair $ \{i, j\} $ with $ i < j $. That is: $$ \sum_{1 \leq i < j \leq n} h_i \cdot h_j $$

API Response (JSON)

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