#include <iostream>
#include <algorithm>
#include <string>
using namespace std;
 
int main() {
    int n;
    cin>>n;
    long long a[100005];
    for(int i = 0;i <n; i++) {
        cin>>a[i];
    }
    sort(a, a+n);
    unsigned long long ans = 0;
    for(int i = n-1; i>=3; i--) {
        if(a[i] - a[i-1] <= 1 && a[i-2] - a[i-3] <= 1) {
            ans +=  (a[i-1] * a[i-3]);
            i-=3;
        }
    }
    cout<<ans;
    return 0;
}

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MTQKNSA1IDUgNSA1IDUgNCA0IDQgNCAyIDIgMiAy

14
5 5 5 5 5 5 4 4 4 4 2 2 2 2