#!/usr/athena/bin/perl

while(<>) {
    ($x, $y, $z) = split;
    $ct++;
    $xt += $x;
    $yt += $y;
    $zt += $z;
    push(@x, $x);
    push(@x, $y);
    push(@x, $z);
}
for $n (0..$#x){
    $x = $x[$n];
    $y = $y[$n];
    $z = $z[$n];

    $dx = $x-$xa;
    $dy = $y-$ya;
    $dz = $z-$za;
    $dist = sqrt(($dx*$dx)+($dy*$dy)+($dz*$dz));
    $dsum += $dist;
}
print $dsum/($#x+1), "\n";