#include <fstream>
#include <algorithm>
#define NMAX 800

using namespace std;
int v[NMAX],sol,n,j,i,numarul,st,dr,mij;
int main()
{
    ifstream f("nrtri.in");
    ofstream g("nrtri.out");
    f>>n;
    for(i=1;i<=n;i++)
    f>>v[i];
    sort(v, v+n+1);
    for(i=1;i<n;i++)
        for(j=i+1;j<n+1;j++)
        {
        numarul=i+j;
        st=j;
        dr=numarul;
        mij=(st+dr)/2;
        while(dr<=st)
        {
            if(v[mij]>numarul)
                dr=mij-1;
            if(v[mij]==numarul)
                break;
            if(v[mij]<numarul)
                st=mij+1;
            mij=(st+dr)/2;
        }
        sol=mij-j+1;
        }
    g<<sol;
}