void main() {
	const N = readInt;
	auto L = new int[N];
	auto S = new int[N];
	foreach (i; 0 .. N) {
		L[i] = readInt;
		S[i] = readInt;
		S[i]--;
	}
	auto vis = new int[N];
	int ans;
	foreach (i; 0 .. N) if (!vis[i]) {
		int u = i;
		do {
			vis[u] = i + 1;
			ans += L[u];
			u = S[u];
		} while(!vis[u]);
		if (vis[u] == i + 1) {
			int v = u;
			int MIN = 100;
			do {
				chmin(MIN, L[u]);
				u = S[u];
			} while (u != v);
			ans += MIN;
		}
	}
	writeln(ans / 2, ans % 2 ? ".5" : ".0");
}

import std,core.bitop;
string[]_R;
string readString(){while(_R.empty){_R=readln.chomp.split;}auto ret=_R.front;_R.popFront;return ret;}
int readInt(){return readString.to!int;}
long readLong(){return readString.to!long;}
ulong readULong(){return readString.to!ulong;}
real readReal(){return readString.to!real;}
bool chmin(T)(ref T A,T B){if(A>B){A=B;return true;}else{return false;}}
bool chmax(T)(ref T A,T B){if(A<B){A=B;return true;}else{return false;}}
int lowerBound(T)(T[]A,T x){int L=-1,R=cast(int)A.length;while(R-L>1){int mid=(L+R)/2;(A[mid]<x?L:R)=mid;}return R;}
int upperBound(T)(T[]A,T x){int L=-1,R=cast(int)A.length;while(R-L>1){int mid=(L+R)/2;(A[mid]<=x?L:R)=mid;}return R;}