import java.io.*;
import java.util.*;

@SuppressWarnings("unused")
public class Main {
    private FastScanner in;
    private PrintWriter out;
    final int MOD = (int)1e9+7;
    long ceil(long a, long b){return (a + b - 1) / b;}
    long gcd(long a, long b){return b == 0 ? a : gcd(b, a % b);}
    long lcm(long a, long b){return a / gcd(a, b) * b; /*オーバーフローに注意*/}

    void solve(){
        int N = in.nextInt();
        ArrayList<ArrayList<Integer>> g = new ArrayList<>();
        for(int i = 0; i < N; i++) g.add(new ArrayList<>());
        double[] L = new double[N];
        for (int i = 0; i < N; i++) {
            L[i] = in.nextDouble();
            int S = in.nextInt()-1;
            g.get(S).add(i);
        }
        StronglyConnectedComponents scc = new StronglyConnectedComponents(g);
        boolean[] visited = new boolean[scc.size];
        double ans = 0;
        for(int i = 0; i < scc.size; i++){
            double min = Double.MAX_VALUE, sum = 0;
            for(int v : scc.vs.get(i)){
                min = Math.min(min, L[v]);
                sum += L[v];
            }
            if(!visited[i]) sum += min;
            sum /= 2;
            ans += sum;
            for(int to : scc.cg.get(i)) visited[to] = true;
        }
        out.println(ans);
    }
    //end solve

    static class StronglyConnectedComponents {
        //size of contracted graph
        int size;
        //vertex's index in contracted graph
        int[] comp;
        //contracted graph, vertices that belong vertex of contracted graph
        ArrayList<ArrayList<Integer>> cg, vs;

        private ArrayList<ArrayList<Integer>> g, gg, rg;
        private ArrayList<Integer> order;
        private boolean[] used;

        StronglyConnectedComponents(ArrayList<ArrayList<Integer>> g) {
            this.g = g;
            gg = new ArrayList<>();
            rg = new ArrayList<>();
            for(int i = 0; i < g.size(); i++) gg.add(new ArrayList<>());
            for(int i = 0; i < g.size(); i++) rg.add(new ArrayList<>());
            used = new boolean[g.size()];
            comp = new int[g.size()];
            Arrays.fill(comp, -1);

            for(int from = 0; from < g.size(); from++){
                for(int to : g.get(from)){
                    gg.get(from).add(to);
                    rg.get(to).add(from);
                }
            }

            build();
            buildGraph();
            buildSets();
        }

        private void build(){
            order = new ArrayList<>();
            for (int i = 0; i < gg.size(); i++){
                dfs(i);
            }
            Collections.reverse(order);
            int ptr = 0;
            for (int i : order){
                if(comp[i] == -1){
                    reverseDfs(i, ptr);
                    ptr++;
                }
            }
            size = ptr;
        }

        private void dfs(int idx) {
            if(used[idx]) return;
            used[idx] = true;
            for (int to : gg.get(idx)) dfs(to);
            order.add(idx);
        }

        private void reverseDfs(int idx, int cnt) {
            if(comp[idx] != -1) return;
            comp[idx] = cnt;
            for (int to : rg.get(idx)) reverseDfs(to, cnt);
        }

        private void buildGraph(){
            cg = new ArrayList<>();
            for(int i = 0; i < size; i++) cg.add(new ArrayList<>());
            for(int i = 0; i < g.size(); i++){
                for(int to : g.get(i)){
                    int x = comp[i], y = comp[to];
                    if(x == y) continue;
                    cg.get(x).add(y);
                }
            }
        }

        private void buildSets(){
            vs = new ArrayList<>();
            for(int i = 0; i < size; i++) vs.add(new ArrayList<>());
            for(int i = 0; i < g.size(); i++){
                vs.get(comp[i]).add(i);
            }
        }
    }

    public static void main(String[] args) {
        new Main().m();
    }

    private void m() {
        in = new FastScanner(System.in);
        out = new PrintWriter(System.out);
        solve();
        out.flush();
        in.close();
        out.close();
    }

    static class FastScanner {
        private Reader input;

        public FastScanner() {this(System.in);}
        public FastScanner(InputStream stream) {this.input = new BufferedReader(new InputStreamReader(stream));}
        public void close() {
            try {
                this.input.close();
            } catch (IOException e) {
                e.printStackTrace();
            }
        }

        public int nextInt() {return (int) nextLong();}

        public long nextLong() {
            try {
                int sign = 1;
                int b = input.read();
                while ((b < '0' || '9' < b) && b != '-' && b != '+') {
                    b = input.read();
                }
                if (b == '-') {
                    sign = -1;
                    b = input.read();
                } else if (b == '+') {
                    b = input.read();
                }
                long ret = b - '0';
                while (true) {
                    b = input.read();
                    if (b < '0' || '9' < b) return ret * sign;
                    ret *= 10;
                    ret += b - '0';
                }
            } catch (IOException e) {
                e.printStackTrace();
                return -1;
            }
        }

        public double nextDouble() {
            try {
                double sign = 1;
                int b = input.read();
                while ((b < '0' || '9' < b) && b != '-' && b != '+') {
                    b = input.read();
                }
                if (b == '-') {
                    sign = -1;
                    b = input.read();
                } else if (b == '+') {
                    b = input.read();
                }
                double ret = b - '0';
                while (true) {
                    b = input.read();
                    if (b < '0' || '9' < b) break;
                    ret *= 10;
                    ret += b - '0';
                }
                if (b != '.') return sign * ret;
                double div = 1;
                b = input.read();
                while ('0' <= b && b <= '9') {
                    ret *= 10;
                    ret += b - '0';
                    div *= 10;
                    b = input.read();
                }
                return sign * ret / div;
            } catch (IOException e) {
                e.printStackTrace();
                return Double.NaN;
            }
        }

        public char nextChar() {
            try {
                int b = input.read();
                while (Character.isWhitespace(b)) {
                    b = input.read();
                }
                return (char) b;
            } catch (IOException e) {
                e.printStackTrace();
                return 0;
            }
        }

        public String nextStr() {
            try {
                StringBuilder sb = new StringBuilder();
                int b = input.read();
                while (Character.isWhitespace(b)) {
                    b = input.read();
                }
                while (b != -1 && !Character.isWhitespace(b)) {
                    sb.append((char) b);
                    b = input.read();
                }
                return sb.toString();
            } catch (IOException e) {
                e.printStackTrace();
                return "";
            }
        }

        public int[] nextIntArray(int n) {
            int[] res = new int[n];
            for (int i = 0; i < n; i++) {
                res[i] = nextInt();
            }
            return res;
        }

        public int[] nextIntArrayDec(int n) {
            int[] res = new int[n];
            for (int i = 0; i < n; i++) {
                res[i] = nextInt() - 1;
            }
            return res;
        }

        public int[] nextIntArray1Index(int n) {
            int[] res = new int[n + 1];
            for (int i = 0; i < n; i++) {
                res[i + 1] = nextInt();
            }
            return res;
        }

        public long[] nextLongArray(int n) {
            long[] res = new long[n];
            for (int i = 0; i < n; i++) {
                res[i] = nextLong();
            }
            return res;
        }

        public long[] nextLongArrayDec(int n) {
            long[] res = new long[n];
            for (int i = 0; i < n; i++) {
                res[i] = nextLong() - 1;
            }
            return res;
        }

        public long[] nextLongArray1Index(int n) {
            long[] res = new long[n + 1];
            for (int i = 0; i < n; i++) {
                res[i + 1] = nextLong();
            }
            return res;
        }

        public double[] nextDoubleArray(int n) {
            double[] res = new double[n];
            for (int i = 0; i < n; i++) {
                res[i] = nextDouble();
            }
            return res;
        }
    }
}