#include <bits/stdc++.h>
using namespace std;

typedef long long   signed int LL;
typedef long long unsigned int LU;

#define incID(i, l, r) for(int i = (l)    ; i <  (r); i++)
#define incII(i, l, r) for(int i = (l)    ; i <= (r); i++)
#define decID(i, l, r) for(int i = (r) - 1; i >= (l); i--)
#define decII(i, l, r) for(int i = (r)    ; i >= (l); i--)
#define  inc(i, n) incID(i, 0, n)
#define inc1(i, n) incII(i, 1, n)
#define  dec(i, n) decID(i, 0, n)
#define dec1(i, n) decII(i, 1, n)

#define inII(v, l, r) ((l) <= (v) && (v) <= (r))
#define inID(v, l, r) ((l) <= (v) && (v) <  (r))

#define PB push_back
#define EB emplace_back
#define MP make_pair
#define FI first
#define SE second
#define UB upper_bound
#define LB lower_bound
#define PQ priority_queue

#define  ALL(v)  v.begin(),  v.end()
#define RALL(v) v.rbegin(), v.rend()
#define  FOR(it, v) for(auto it =  v.begin(); it !=  v.end(); ++it)
#define RFOR(it, v) for(auto it = v.rbegin(); it != v.rend(); ++it)

template<typename T> bool   setmin(T & a, T b) { if(b <  a) { a = b; return true; } else { return false; } }
template<typename T> bool   setmax(T & a, T b) { if(b >  a) { a = b; return true; } else { return false; } }
template<typename T> bool setmineq(T & a, T b) { if(b <= a) { a = b; return true; } else { return false; } }
template<typename T> bool setmaxeq(T & a, T b) { if(b >= a) { a = b; return true; } else { return false; } }
template<typename T> T gcd(T a, T b) { return (b == 0 ? a : gcd(b, a % b)); }
template<typename T> T lcm(T a, T b) { return a / gcd(a, b) * b; }

// ---- ----

int n, l[100], s[100], ans, d[100];

int main() {
	cin >> n;
	inc(i, n) { cin >> l[i] >> s[i]; }
	
	inc(i, n) { ans += l[i]; s[i]--; d[s[i]]++; }
	
	queue<int> q;
	bool flag = true;
	while(true) {
		if(flag) {
			inc(i, n) { if(d[i] == 0) { q.push(i); } }
			flag = false;
		} else {
			int mi = 1001, p = -1;
			inc(i, n) { if(d[i] > 0 && setmin(mi, l[i])) { p = i; } }
			if(p == -1) { break; }
			ans += l[p];
			q.push(p);
		}
		while(! q.empty()) {
			int e = q.front(); q.pop();
			d[e] = 0;
			d[s[e]]--;
			if(d[s[e]] == 0) { q.push(s[e]); }
		}
	}
	
	printf("%.1f\n", ans / 2.0);
	
	return 0;
}