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

typedef long long ll;
typedef vector< int > vi;
typedef vector< vi > vvi;
typedef vector< ll > vl;
typedef vector< vl > vvl;
typedef pair< int, int > pii;
typedef vector< pii > vp;
typedef vector< double > vd;
typedef vector< vd > vvd;
typedef vector< string > vs;

template< class T1, class T2 >
int upmin( T1 &x, T2 v ){
  if( x > v ){
    x = v;
    return 1;
  }
  return 0;
}

template< class T1, class T2 >
int upmax( T1 &x, T2 v ){
  if( x < v ){
    x = v;
    return 1;
  }
  return 0;
}

const int INF = 0x3f3f3f3f;

int N;
vi L, S;

void init(){
  cin >> N;
  L = S = vi( N );
  for( int i = 0; i < N; ++i ){
    cin >> L[ i ] >> S[ i ];
    --S[ i ];
  }
}

double ans;

void preprocess(){
  for( int i = 0; i < N; ++i ){
    ans += 1.0 * L[ i ] / 2;
  }
  vi vis( N );
  for( int i = 0; i < N; ++i ){
    if( vis[ i ] ) continue;
    int u;
    vi cur_vis( N );
    int ng = 0;
    for( u = i; not cur_vis[ u ]; u = S[ u ] ){
      if( vis[ u ] ){
        ng = 1;
        break;
      }
      vis[ u ] = 1;
      cur_vis[ u ] = 1;
    }
    if( ng ) continue;
    int minv = L[ u ];
    for( int v = S[ u ]; v != u; v = S[ v ] ){
      vis[ v ] = 1;
      upmin( minv, L[ v ] );
    }
    ans += 1.0 * minv / 2;
  }
}

void solve(){
  cout << fixed << setprecision( 1 ) << ans << endl;
}

signed main(){
  ios::sync_with_stdio( 0 );
  init();
  preprocess();
  solve();
  return 0;
}