#!/usr/bin/awk

BEGIN{
    FS = "\t"
}

{
    tab1[NR] = $1;
    tab2[NR] = $2;
}

END{
    commun = 0;
    diff = 0;
    for (i = 1; i <= NR; i ++)
	for (j = i + 1; j <= NR; j ++){
	    if (tab1[i] == tab1[j] && tab2[i] == tab2[j]){
		#print "commun"
		commun ++;
	    }
	    if (tab1[i] != tab1[j] && tab2[i] != tab2[j]){
		#print "diff"
		diff ++;
	    }
	    #print i, tab1[i], tab2[i], "-", j, tab1[j], tab2[j]
	}
    print (commun + diff) / (NR * (NR - 1) / 2)
}