Parallelograms

pll R[MX*MX];
ll par(ll N,ll*x,ll*y){
    ll L(0),A(0),c(0);
    F(N)FT(i+1,N)R[L++]={x[i]+x[k],y[i]+y[k]};
    sort(R,R+L);
    F(L-1)if(R[i+1]==R[i])++c;
          else A+=c*(c+1)/2,c=0;
    return A;
}