#include #include #include #include #include #include #include #include #include #include #include #include #include #include #include #include #include #include #include using namespace std; typedef long long ll; typedef unsigned int ui; const ll mod = 1000000007; const ll INF = (ll)1000000007 * 1000000007; typedef pair P; #define stop char nyaa;cin>>nyaa; #define rep(i,n) for(int i=0;i=0;i--) #define Rep(i,sta,n) for(int i=sta;i=sta;i--) #define rep1(i,n) for(int i=1;i<=n;i++) #define per1(i,n) for(int i=n;i>=1;i--) #define Rep1(i,sta,n) for(int i=sta;i<=n;i++) typedef long double ld; typedef complex Point; const ld eps = 1e-8; const ld pi = acos(-1.0); typedef pair LP; struct SCC{ vector > G,R,T,C;//T:強連結分解後のグラフ vector vs,used,blg; SCC(int n):G(n),R(n),used(n),blg(n){} void add_edge(int u,int v){ G[u].emplace_back(v); R[v].emplace_back(u); } void dfs(int v){ used[v]=1; for(int u:G[v]) if(!used[u]) dfs(u); vs.emplace_back(v); } void rdfs(int v,int k){ used[v]=1; blg[v]=k; C[k].emplace_back(v); for(int u:R[v]) if(!used[u]) rdfs(u,k); } int build(){ int n=G.size(); for(int v=0;v=0;i--){ if(!used[vs[i]]){ T.emplace_back(); C.emplace_back(); rdfs(vs[i],k++); } } for(int v=0;v se[110];; void solve(){ cin >> n; SCC scc(n); ld ans=0; rep(i,n){ int j; cin >> c[i] >> j; scc.add_edge(i,j-1); ans+=c[i]/2; } int k=scc.build(); //cout << k << endl; rep(i,n){ //cout << i << " " << scc[i] << endl; se[scc[i]].insert(c[i]); } rep(u,k){ if (scc.T[u].size()==0) ans+=(*se[u].begin())/2; } cout << ans << endl; } int main(){ ios::sync_with_stdio(false); cin.tie(0); cout << fixed << setprecision(50); solve(); }