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