l[999],s[999],f[999],u,v,a,b;main(n){scanf("%d",&n);for(u=0;u++<n;)scanf("%d%d",l+u,s+u);for(u=0;u++<n;){a+=l[v=u];for(b=n;b--;v=s[v]);for(b=999;!f[v]++;v=s[v])b=b>l[v]?l[v]:b;a+=b<999?b:0;}printf("%.1f",a/2.);}