L[99],S[99],v[99],e,s,p,m,x;f(i){v[i]||(v[i]=p,x+=m=L[i],v[s=S[i]]==p?e=s:f(s));L[i]<m?m=L[i]:0;x+=i-e?0:m;}main(i){for(gets(L);~scanf("%d%d",L+i,S+i);++i);for(;--i;f(i))e=!++p;printf("%.1f",x/2.);}