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

class SCC {

	static class Edge {
		int from, to;

		public Edge(int from, int to) {
			this.from = from;
			this.to = to;
		}
	}

	final int n;
	int m;
	final java.util.ArrayList<Edge> unorderedEdges;
	final int[] start;
	final int[] ids;
	boolean hasBuilt = false;

	public SCC(int n) {
		this.n = n;
		this.unorderedEdges = new java.util.ArrayList<>();
		this.start = new int[n + 1];
		this.ids = new int[n];
	}

	public void addEdge(int from, int to) {
		rangeCheck(from);
		rangeCheck(to);
		unorderedEdges.add(new Edge(from, to));
		start[from + 1]++;
		this.m++;
	}

	public int id(int i) {
		if (!hasBuilt) {
			throw new UnsupportedOperationException("Graph hasn't been built.");
		}
		rangeCheck(i);
		return ids[i];
	}

	public int[][] build() {
		for (int i = 1; i <= n; i++) {
			start[i] += start[i - 1];
		}
		Edge[] orderedEdges = new Edge[m];
		int[] count = new int[n + 1];
		System.arraycopy(start, 0, count, 0, n + 1);
		for (Edge e : unorderedEdges) {
			orderedEdges[count[e.from]++] = e;
		}
		int nowOrd = 0;
		int groupNum = 0;
		int k = 0;
		// parent
		int[] par = new int[n];
		int[] vis = new int[n];
		int[] low = new int[n];
		int[] ord = new int[n];
		java.util.Arrays.fill(ord, -1);
		// u = lower32(stack[i]) : visiting vertex
		// j = upper32(stack[i]) : jth child
		long[] stack = new long[n];
		// size of stack
		int ptr = 0;
		// non-recursional DFS
		for (int i = 0; i < n; i++) {
			if (ord[i] >= 0)
				continue;
			par[i] = -1;
			// vertex i, 0th child.
			stack[ptr++] = 0l << 32 | i;
			// stack is not empty
			while (ptr > 0) {
				// last element
				long p = stack[--ptr];
				// vertex
				int u = (int) (p & 0xffff_ffffl);
				// jth child
				int j = (int) (p >>> 32);
				if (j == 0) { // first visit
					low[u] = ord[u] = nowOrd++;
					vis[k++] = u;
				}
				if (start[u] + j < count[u]) { // there are more children
					// jth child
					int to = orderedEdges[start[u] + j].to;
					// incr children counter
					stack[ptr++] += 1l << 32;
					if (ord[to] == -1) { // new vertex
						stack[ptr++] = 0l << 32 | to;
						par[to] = u;
					} else { // backward edge
						low[u] = Math.min(low[u], ord[to]);
					}
				} else { // no more children (leaving)
					while (j-- > 0) {
						int to = orderedEdges[start[u] + j].to;
						// update lowlink
						if (par[to] == u)
							low[u] = Math.min(low[u], low[to]);
					}
					if (low[u] == ord[u]) { // root of a component
						while (true) { // gathering verticies
							int v = vis[--k];
							ord[v] = n;
							ids[v] = groupNum;
							if (v == u)
								break;
						}
						groupNum++; // incr the number of components
					}
				}
			}
		}
		for (int i = 0; i < n; i++) {
			ids[i] = groupNum - 1 - ids[i];
		}

		int[] counts = new int[groupNum];
		for (int x : ids)
			counts[x]++;
		int[][] groups = new int[groupNum][];
		for (int i = 0; i < groupNum; i++) {
			groups[i] = new int[counts[i]];
		}
		for (int i = 0; i < n; i++) {
			int cmp = ids[i];
			groups[cmp][--counts[cmp]] = i;
		}
		hasBuilt = true;
		return groups;
	}

	private void rangeCheck(int i) {
		if (i < 0 || i >= n) {
			throw new IndexOutOfBoundsException(String.format("Index %d out of bounds for length %d", i, n));
		}
	}
}

public class Main {
	static void solve() {
		int n = ni();
		var scc = new SCC(n);
		List<Integer>[] next = new ArrayList[n];
		for(int i=0;i<n;i++) next[i] = new ArrayList<>();
		double[] l = new double[n];
		for(int i=0;i<n;i++) {
			l[i] = ni();
			int from = ni()-1;
			next[from].add(i);
			scc.addEdge(from, i);
		}
		var res = scc.build();
		double ans = 0;
		boolean[] clear = new boolean[n];

		for(int i=0;i<res.length;i++) {
			double min = linf;
			boolean ok = false;
			
			for(int t : res[i]) {
				if(clear[t])ok = true;
			}
			
			for(int t : res[i]) {
				min = min(min, l[t]);
				ans += l[t];
				for(int to : next[t]) {
					clear[to] = true;					
				}
			}
			if(!ok)ans += min;
		}

		ans /= 2;
		out.println(ans);
	}

	//constants
	static final int inf = Integer.MAX_VALUE / 2;
	static final long linf = Long.MAX_VALUE / 3;
	static final double dinf = Double.MAX_VALUE / 3;
	static final long mod = (long) 1e9 + 7;
	static final int[] dx = { -1, 0, 1, 0 }, dy = { 0, -1, 0, 1 }, dx8 = { -1, -1, -1, 0, 0, 1, 1, 1 },
			dy8 = { -1, 0, 1, -1, 1, -1, 0, 1 };
	static final double eps = 1e-10;

	//libraries
	static long[] cum(int a[]) {
		long[] cum = new long[a.length + 1];
		for (int i = 0; i < a.length; i++)
			cum[i + 1] = cum[i] + a[i];
		return cum;
	}

	static long[] cum(long a[]) {
		long[] cum = new long[a.length + 1];
		for (int i = 0; i < a.length; i++)
			cum[i + 1] = cum[i] + a[i];
		return cum;
	}

	static void reverse(int ar[]) {
		int len = ar.length;
		for (int i = 0; i < len / 2; i++) {
			int t = ar[i];
			ar[i] = ar[len - 1 - i];
			ar[len - 1 - i] = t;
		}
	}

	static void reverse(long ar[]) {
		int len = ar.length;
		for (int i = 0; i < len / 2; i++) {
			long t = ar[i];
			ar[i] = ar[len - 1 - i];
			ar[len - 1 - i] = t;
		}
	}

	static void reverse(double ar[]) {
		int len = ar.length;
		for (int i = 0; i < len / 2; i++) {
			double t = ar[i];
			ar[i] = ar[len - 1 - i];
			ar[len - 1 - i] = t;
		}
	}

	static void reverse(char ar[]) {
		int len = ar.length;
		for (int i = 0; i < len / 2; i++) {
			char t = ar[i];
			ar[i] = ar[len - 1 - i];
			ar[len - 1 - i] = t;
		}
	}

	static String getReverse(String s) {
		StringBuilder sb = new StringBuilder(s);
		return sb.reverse().toString();
	}

	static <T> void reverse(T[] ar) {
		int len = ar.length;
		for (int i = 0; i < len / 2; i++) {
			T t = ar[i];
			ar[i] = ar[len - 1 - i];
			ar[len - 1 - i] = t;
		}
	}

	static int[] concat(int x, int arr[]) {
		int ret[] = new int[arr.length + 1];
		System.arraycopy(arr, 0, ret, 1, ret.length - 1);
		ret[0] = x;
		return ret;
	}

	static int[] concat(int arr[], int x) {
		int ret[] = new int[arr.length + 1];
		System.arraycopy(arr, 0, ret, 0, ret.length - 1);
		ret[ret.length - 1] = x;
		return ret;
	}

	static long[] concat(long x, long arr[]) {
		long ret[] = new long[arr.length + 1];
		System.arraycopy(arr, 0, ret, 1, ret.length - 1);
		ret[0] = x;
		return ret;
	}

	static long[] concat(long arr[], long x) {
		long ret[] = new long[arr.length + 1];
		System.arraycopy(arr, 0, ret, 0, ret.length - 1);
		ret[ret.length - 1] = x;
		return ret;
	}

	static char[] concat(char x, char arr[]) {
		char ret[] = new char[arr.length + 1];
		System.arraycopy(arr, 0, ret, 0, ret.length - 1);
		ret[ret.length - 1] = x;
		return ret;
	}

	static char[] concat(char arr[], char x) {
		char ret[] = new char[arr.length + 1];
		System.arraycopy(arr, 0, ret, 0, ret.length - 1);
		ret[ret.length - 1] = x;
		return ret;
	}

	static int max(int x, int y) {
		return Math.max(x, y);
	}

	static int min(int x, int y) {
		return Math.min(x, y);
	}

	static int max(int x, int y, int z) {
		x = Math.max(x, y);
		x = Math.max(x, z);
		return x;
	}

	static int min(int x, int y, int z) {
		x = Math.min(x, y);
		x = Math.min(x, z);
		return x;
	}

	static long max(long x, long y) {
		return Math.max(x, y);
	}

	static long min(long x, long y) {
		return Math.min(x, y);
	}

	static long max(long x, long y, long z) {
		x = Math.max(x, y);
		x = Math.max(x, z);
		return x;
	}

	static long min(long x, long y, long z) {
		x = Math.min(x, y);
		x = Math.min(x, z);
		return x;
	}

	static double max(double x, double y) {
		return Math.max(x, y);
	}

	static double min(double x, double y) {
		return Math.min(x, y);
	}

	static double max(double x, double y, double z) {
		x = Math.max(x, y);
		x = Math.max(x, z);
		return x;
	}

	static double min(double x, double y, double z) {
		x = Math.min(x, y);
		x = Math.min(x, z);
		return x;
	}

	static void sort(int[] ar) {
		Arrays.sort(ar);
	}

	static void sort(long[] ar) {
		Arrays.sort(ar);
	}

	static void sort(double[] ar) {
		Arrays.sort(ar);
	}

	static void sort(char[] ar) {
		Arrays.sort(ar);
	}

	static void rsort(int[] ar) {
		Arrays.sort(ar);
		int len = ar.length;
		for (int i = 0; i < len / 2; i++) {
			int tmp = ar[i];
			ar[i] = ar[len - 1 - i];
			ar[len - 1 - i] = tmp;
		}
	}

	static void rsort(long[] ar) {
		Arrays.sort(ar);
		int len = ar.length;
		for (int i = 0; i < len / 2; i++) {
			long tmp = ar[i];
			ar[i] = ar[len - 1 - i];
			ar[len - 1 - i] = tmp;
		}
	}

	static void rsort(double[] ar) {
		Arrays.sort(ar);
		int len = ar.length;
		for (int i = 0; i < len / 2; i++) {
			double tmp = ar[i];
			ar[i] = ar[len - 1 - i];
			ar[len - 1 - i] = tmp;
		}
	}

	static void rsort(char[] ar) {
		Arrays.sort(ar);
		int len = ar.length;
		for (int i = 0; i < len / 2; i++) {
			char tmp = ar[i];
			ar[i] = ar[len - 1 - i];
			ar[len - 1 - i] = tmp;
		}
	}

	static void fill(int arr[], int x) {
		Arrays.fill(arr, x);
	}

	static void fill(long arr[], long x) {
		Arrays.fill(arr, x);
	}

	static void fill(boolean arr[], boolean x) {
		Arrays.fill(arr, x);
	}

	static void fill(double arr[], double x) {
		Arrays.fill(arr, x);
	}

	static void fill(int arr[][], int x) {
		for (int i = 0; i < arr.length; i++)
			Arrays.fill(arr[i], x);
	}

	static void fill(long arr[][], long x) {
		for (int i = 0; i < arr.length; i++)
			Arrays.fill(arr[i], x);
	}

	static void fill(double arr[][], double x) {
		for (int i = 0; i < arr.length; i++)
			Arrays.fill(arr[i], x);
	}

	static void fill(boolean arr[][], boolean x) {
		for (int i = 0; i < arr.length; i++)
			Arrays.fill(arr[i], x);
	}

	//MOD culc
	static long plus(long x, long y) {
		long res = (x + y) % mod;
		return res < 0 ? res + mod : res;
	}

	static long sub(long x, long y) {
		long res = (x - y) % mod;
		return res < 0 ? res + mod : res;
	}

	static long mul(long x, long y) {
		long res = (x * y) % mod;
		return res < 0 ? res + mod : res;
	}

	static long div(long x, long y) {
		long res = x * pow(y, mod - 2) % mod;
		return res < 0 ? res + mod : res;
	}

	static long pow(long x, long y) {
		if (y < 0)
			return 0;
		if (y == 0)
			return 1;
		if (y % 2 == 1)
			return (x * pow(x, y - 1)) % mod;
		long root = pow(x, y / 2);
		return root * root % mod;
	}

	public static void main(String[] args) throws Exception {
		is = INPUT.isEmpty() ? System.in : new ByteArrayInputStream(INPUT.getBytes());
		out = new PrintWriter(System.out);
		solve();
		out.flush();
	}

	//input
	static InputStream is;
	static PrintWriter out;
	static String INPUT = "";
	private static byte[] inbuf = new byte[1024];
	static int lenbuf = 0, ptrbuf = 0;

	private static int readByte() {
		if (lenbuf == -1)
			throw new InputMismatchException();
		if (ptrbuf >= lenbuf) {
			ptrbuf = 0;
			try {
				lenbuf = is.read(inbuf);
			} catch (IOException e) {
				throw new InputMismatchException();
			}
			if (lenbuf <= 0)
				return -1;
		}
		return inbuf[ptrbuf++];
	}

	private static boolean isSpaceChar(int c) {
		return !(c >= 33 && c <= 126);
	}

	private static int skip() {
		int b;
		while ((b = readByte()) != -1 && isSpaceChar(b))
			;
		return b;
	}

	@SuppressWarnings("unused")
	private static double nd() {
		return Double.parseDouble(ns());
	}

	@SuppressWarnings("unused")
	private static char nc() {
		return (char) skip();
	}

	private static String ns() {
		int b = skip();
		StringBuilder sb = new StringBuilder();
		while (!(isSpaceChar(b))) {
			sb.appendCodePoint(b);
			b = readByte();
		}
		return sb.toString();
	}

	private static char[] ns(int n) {
		char[] buf = new char[n];
		int b = skip(), p = 0;
		while (p < n && !(isSpaceChar(b))) {
			buf[p++] = (char) b;
			b = readByte();
		}
		return n == p ? buf : Arrays.copyOf(buf, p);
	}

	@SuppressWarnings("unused")
	private static char[][] nm(int n, int m) {
		char[][] map = new char[n][];
		for (int i = 0; i < n; i++)
			map[i] = ns(m);
		return map;
	}

	@SuppressWarnings("unused")
	private static int[] na(int n) {
		int[] a = new int[n];
		for (int i = 0; i < n; i++)
			a[i] = ni();
		return a;
	}

	@SuppressWarnings("unused")
	private static long[] nla(int n) {
		long[] a = new long[n];
		for (int i = 0; i < n; i++)
			a[i] = nl();
		return a;
	}

	@SuppressWarnings("unused")
	private static int[][] na(int n, int m) {
		int[][] res = new int[n][m];
		for (int i = 0; i < n; i++) {
			for (int j = 0; j < m; j++) {
				res[i][j] = ni();
			}
		}
		return res;
	}

	@SuppressWarnings("unused")
	private static long[][] nla(int n, int m) {
		long[][] res = new long[n][m];
		for (int i = 0; i < n; i++) {
			for (int j = 0; j < m; j++) {
				res[i][j] = nl();
			}
		}
		return res;
	}

	private static int ni() {
		int num = 0, b;
		boolean minus = false;
		while ((b = readByte()) != -1 && !((b >= '0' && b <= '9') || b == '-'))
			;
		if (b == '-') {
			minus = true;
			b = readByte();
		}

		while (true) {
			if (b >= '0' && b <= '9') {
				num = num * 10 + (b - '0');
			} else {
				return minus ? -num : num;
			}
			b = readByte();
		}
	}

	private static long nl() {
		long num = 0;
		int b;
		boolean minus = false;
		while ((b = readByte()) != -1 && !((b >= '0' && b <= '9') || b == '-'))
			;
		if (b == '-') {
			minus = true;
			b = readByte();
		}

		while (true) {
			if (b >= '0' && b <= '9') {
				num = num * 10 + (b - '0');
			} else {
				return minus ? -num : num;
			}
			b = readByte();
		}
	}
}