#include <stack>
#include <stdio.h>
#include <list>
#include <cassert>
#include <set>
#include <iostream>
#include <string>
#include <vector>
#include <queue>
#include <functional>
#include <cstring>
#include <algorithm>
#include <cctype>
#include <string>
#include <map>
#include <cmath>
using namespace std;
#define LL long long
#define ULL unsigned long long
#define SZ(x) (int)x.size()
#define Lowbit(x) ((x) & (-x))
#define MP(a, b) make_pair(a, b)
#define MS(arr, num) memset(arr, num, sizeof(arr))
#define PB push_back
#define X first
#define Y second
#define ROP freopen("input.txt", "r", stdin);
#define MID(a, b) (a + ((b - a) >> 1))
#define LC rt << 1, l, mid
#define RC rt << 1|1, mid + 1, r
#define LRT rt << 1
#define RRT rt << 1|1
const double PI = acos(-1.0);
const int INF = 0x3f3f3f3f;
const double eps = 1e-8;
const int MAXN = 1e6 + 10;
const int MOD = 9901;
const int dir[][2] = { {-1, 0}, {1, 0}, {0, -1}, {0, 1} };
int cases = 0;
typedef pair<int, int> pii;
 
int l[MAXN];
 
//适用于正负数,(int,long long,float,double)
template <class T>
bool Read(T &ret)
{
    char c; int sgn; T bit=0.1;
    if(c=getchar(),c==EOF) return 0;
    while(c!='-'&&c!='.'&&(!isdigit(c))) c=getchar();
    sgn=(c=='-')?-1:1;
    ret=(c=='-')?0:(c-'0');
    while(c=getchar(),isdigit(c)) ret=ret*10+(c-'0');
    if(c==' '||c=='\n'){ ret*=sgn; return 1; }
    while(c=getchar(),isdigit(c)) ret+=(c-'0')*bit,bit/=10;
    ret*=sgn;
    return 1;
}
 
int GetLong()
{
    LL last = 0, ans = 0;
    for (LL i = MAXN-1; i >= 2; i--)
    {
        int cnt = 0;
        if (l[i])
        {
            if (l[i+1])
            {
                cnt++;
                l[i]--;
            }
            cnt += l[i]/2;
            if (cnt && last)
            {
                ans += last*i, cnt--;
                last = 0;
            }
            l[i] = l[i] % 2;
            ans += i*i*(cnt>>1);
            if (cnt&1) last = i;
        }
    }
    printf("%I64d\n", ans);
}
 
int main()
{
    //ROP;
    int n;
    scanf("%d", &n);
    for (int i = 0; i < n; i++)
    {
        int tmp;
        Read(tmp);
        l[tmp]++;
    }
    GetLong();
    return 0;
}