#include <map>
#include <set>
#include <list>
#include <cmath>
#include <ctime>
#include <deque>
#include <queue>
#include <stack>
#include <string>
#include <bitset>
#include <cstdio>
#include <limits>
#include <vector>
#include <climits>
#include <cstring>
#include <cstdlib>
#include <fstream>
#include <numeric>
#include <sstream>
#include <cassert>
#include <iomanip>
#include <iostream>
#include <algorithm>
#include <stdio.h>
#include <fstream>
#include <unordered_map>
using namespace std;
typedef long long ll;
typedef long double ld;
#define all(x) x.begin(),x.end()
#define pb push_back
#define ent "\n"
const int maxn = (int)3e5 + 13;
const ll inf = (long long)1e18 + 20;
const int mod = (int)1e9 + 7;
int n;
ld a[maxn],b[maxn];
ld pref[maxn];
ld suf[maxn];
int get(int x,int y){
int ans = x / pow(4,y);
ans %= 4;
return ans;
}
void solve(){
cin >> n;
for(int i = 1 ; i <= n ; i ++){
cin >> a[i] >> b[i];
}
sort(a + 1,a + n + 1);
sort(b + 1,b + n + 1);
for(int i = 1 ; i <= n ; i ++){
pref[i] = pref[i - 1] + b[i];
}
ld mx =0 ,sum = 0,sum1 = 0;
int ind = n;
for(int i = n ; i >= 1 ; i --){
sum += a[i];
sum1 = 0;
for(int j = ind ; j >= 1 ; j --){
sum1 = pref[n] - pref[j - 1];
int kol = (n - i + 1) + (n - j + 1);
mx = max(mx,min(sum1,sum) - kol);
if(sum1 > sum){
ind = j;
break;
}
}
}
printf("%.4lf",(double)mx);
}
int main(){
ios_base::sync_with_stdio(0);
cin.tie(0);
int t = 1;
//cin >> t;
while(t --){
solve();
}
return 0;
}
