525C - Ilya and Sticks - CodeForces Solution

from collections import Counter
import sys 
input = sys.stdin.readline
n = int(input()) 
a = [int(i) for i in input().split()] 

a.sort(reverse = True)
cnt = Counter(a) 
l = [] 
for i in cnt : 
    if cnt[i]%2 : 
        if cnt[i-1]>0 : cnt[i-1]+=1; cnt[i]-=1 
        else : cnt[i]-=1 
    if cnt[i]>1 : l.append(i) 
l.sort(reverse = True) 
ans = 0
for i in range(len(l)-1): 
    ans+=cnt[l[i]]//4*(l[i]**2) 
    q = (cnt[l[i]]%4)//2
    ans+=(l[i]*l[i+1])*q
    cnt[l[i+1]] -= q
if l == [] : print(0)
elif cnt[l[-1]] >= 4 : ans+=cnt[l[-1]]//4*(l[-1]**2) ; print(ans)
else : print(ans)