Codeforces #297 (Div. 2) C. Ilya and Sticks（贪心

C. Ilya and Sticks

time limit per test2 secondsmemory limit per test256 megabytesinputstandard inputoutputstandard outputIn the evening, after the contest Ilya was bored, and he really felt like maximizing. He remembered that he had a set of n sticks and an instrument. Each stick is characterized by its length li.
Ilya decided to make a rectangle from the sticks. And due to his whim, he decided to make rectangles in such a way that maximizes their total area. Each stick is used in making at most one rectangle, it is possible that some of sticks remain unused. Bending sticks is not allowed.
Sticks with lengths a1, a2, a3 and a4 can make a rectangle if the following properties are observed:
a1 ≤ a2 ≤ a3 ≤ a4
a1 = a2
a3 = a4
A rectangle can be made of sticks with lengths of, for example, 3 3 3 3 or 2 2 4 4. A rectangle cannot be made of, for example, sticks 5 5 5 7.
Ilya
4000also has an instrument which can reduce the length of the sticks. The sticks are made of a special material, so the length of each stick can be reduced by at most one. For example, a stick with length 5 can either stay at this length or be transformed into a stick of length 4.
You have to answer the question — what maximum total area of the rectangles can Ilya get with a file if makes rectangles from the available sticks?
InputThe first line of the input contains a positive integer n (1 ≤ n ≤ 105) — the number of the available sticks.
The second line of the input contains n positive integers li (2 ≤ li ≤ 106) — the lengths of the sticks.
OutputThe first line of the output must contain a single non-negative integer — the maximum total area of the rectangles that Ilya can make from the available sticks.
ExamplesInput

4
2 4 4 2

Output

8

Input

4
2 2 3 5

Output

0

Input

4
100003 100004 100005 100006

Output

10000800015

很经典的贪心问题，英语太差，看错题系列

注意LL,没理解题意wa一次，在LL这里wa3次 还是菜啊[/u]

#include <cstdio>
#include <cstring>
#include <algorithm>
using namespace std;
#define N 200000+100#define LL long long
bool cmp(int x,int y)
{
return x > y;
}
int arr
;

int main()
{
LL n;
scanf("%lld",&n);
for(int i = 0;i < n; i++) {
scanf("%d",&arr[i]);
}
sort(arr,arr+n,cmp);
LL sum = 0, len = 0;
for(int i = 0;i < n;) {
if(arr[i]==arr[i+1] || arr[i]==arr[i+1]+1) {
if(len == 0) {
len = arr[i+1];
}
else {
sum += arr[i+1]*len;
len = 0;
}
i += 2;
}
else  {
i++;
}
}
printf("%lld",sum);
return 0;
}