using System; using System.Linq; using System.Collections.Generic; using Debug = System.Diagnostics.Debug; using StringBuilder = System.Text.StringBuilder; //using System.Numerics; namespace Program { public class Solver { public void Solve() { var n = sc.Integer(); var val = new double[n]; var G = Enumerate(n, x => new List()); var RG = Enumerate(n, x => new List()); for (int i = 0; i < n; i++) { val[i] = sc.Integer(); var to = sc.Integer() - 1; if (to == i) continue; G[i].Add(to); RG[to].Add(i); } int sz; var scc = DSCCHelper.SCC(G, RG, out sz); var l = Enumerate(sz, x => new List()); for (int i = 0; i < n; i++) l[scc[i]].Add(i); var ans = 0.0; for (int i = sz - 1; i >= 0; i--) { var min = double.MaxValue; foreach (var x in l[i]) { if (val[x] < min) min = val[x]; ans += val[x] / 2; } ans += min / 2; foreach (var x in l[i]) if (G[x].Count > 0) val[G[x][0]] /= 2; } IO.Printer.Out.WriteLine("{0:F1}",ans); } internal IO.StreamScanner sc = new IO.StreamScanner(Console.OpenStandardInput()); static T[] Enumerate(int n, Func f) { var a = new T[n]; for (int i = 0; i < n; ++i) a[i] = f(); return a; } static T[] Enumerate(int n, Func f) { var a = new T[n]; for (int i = 0; i < n; ++i) a[i] = f(i); return a; } } } #region Ex namespace Program.IO { using System.IO; using System.Linq; public class Printer : StreamWriter { static Printer() { Out = new Printer(Console.OpenStandardOutput()) { AutoFlush = false }; } public static Printer Out { get; set; } public override IFormatProvider FormatProvider { get { return System.Globalization.CultureInfo.InvariantCulture; } } public Printer(System.IO.Stream stream) : base(stream, new System.Text.UTF8Encoding(false, true)) { } public void Write(string format, IEnumerable source) { base.Write(format, source.OfType().ToArray()); } public void WriteLine(string format, IEnumerable source) { base.WriteLine(format, source.OfType().ToArray()); } } public class StreamScanner { public StreamScanner(Stream stream) { str = stream; } private readonly Stream str; private readonly byte[] buf = new byte[1024]; private int len, ptr; public bool isEof = false; public bool IsEndOfStream { get { return isEof; } } private byte read() { if (isEof) return 0; if (ptr >= len) { ptr = 0; if ((len = str.Read(buf, 0, 1024)) <= 0) { isEof = true; return 0; } } return buf[ptr++]; } public char Char() { byte b = 0; do b = read(); while (b < 33 || 126 < b); return (char)b; } public char[] Char(int n) { var a = new char[n]; for (int i = 0; i < n; i++) a[i] = Char(); return a; } public string Scan() { var sb = new StringBuilder(); for (var b = Char(); b >= 33 && b <= 126; b = (char)read()) sb.Append(b); return sb.ToString(); } public long Long() { if (isEof) return long.MinValue; long ret = 0; byte b = 0; var ng = false; do b = read(); while (b != '-' && (b < '0' || '9' < b)); if (b == '-') { ng = true; b = read(); } for (; true; b = read()) { if (b < '0' || '9' < b) return ng ? -ret : ret; else ret = ret * 10 + b - '0'; } } public int Integer() { return (isEof) ? int.MinValue : (int)Long(); } public double Double() { return double.Parse(Scan(), System.Globalization.CultureInfo.InvariantCulture); } private T[] enumerate(int n, Func f) { var a = new T[n]; for (int i = 0; i < n; ++i) a[i] = f(); return a; } public string[] Scan(int n) { return enumerate(n, Scan); } public double[] Double(int n) { return enumerate(n, Double); } public int[] Integer(int n) { return enumerate(n, Integer); } public long[] Long(int n) { return enumerate(n, Long); } public void Flush() { str.Flush(); } } } static class Ex { static public string AsString(this IEnumerable ie) { return new string(System.Linq.Enumerable.ToArray(ie)); } static public string AsJoinedString(this IEnumerable ie, string st = " ") { return string.Join(st, ie); } static public void Main() { var solver = new Program.Solver(); solver.Solve(); Program.IO.Printer.Out.Flush(); } } #endregion #region DecomposiitonOfStronglyConnectedComponents static public class DSCCHelper { static public int[] SCC(List[] G, List[] RG, out int k) { var n = G.Length; var used = new bool[n]; var vs = new List(n); var cmp = new int[n]; Action dfs = null; dfs = v => { used[v] = true; foreach (var to in G[v]) if (!used[to]) dfs(to); vs.Add(v); }; Action rdfs = null; rdfs = (v, t) => { used[v] = true; cmp[v] = t; foreach (var to in RG[v]) if (!used[to]) rdfs(to, t); }; for (int i = 0; i < used.Length; i++) if (!used[i]) dfs(i); used = new bool[n]; k = 0; for (int i = vs.Count - 1; i >= 0; i--) if (!used[vs[i]]) rdfs(vs[i], k++); return cmp; } } #endregion