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