//https://oj.uz/problem/view/CEOI17_sure
#define here cerr<<"===========================================\n"
#include <bits/stdc++.h>
#define ld double
#define ll long long
#define llinf 100000000000000000LL // 10^17
#define pb push_back
#define popb pop_back
#define fi first
#define sc second
#define endl '\n'
#define pll pair<ll,ll>
#define pld pair<ld,ld>
#define sz(a) (ll)(a.size())
#define all(a) a.begin(),a.end()
#define ceri(a,l,r) {for(ll i_ = l;i_<=r;i_++) cerr<<a[i_]<< " ";cerr<<endl;}
#define daj_mi_malo_vremena ios_base::sync_with_stdio(false);cerr.tie(0);cout.tie(0);cin.tie(0);
using namespace std;
#define maxn 100005
ll n;
vector<ld> v,w;
bool cmp(ld x,ld y){return x>y;}
ld reshi(vector<ld> a,vector<ld> b){
ld ans = 0;
ld suma = 0;
ld sumb = 0;
ll j = 1;
for(ll i = 1;i<=n;i++){
suma+=a[i];
while(j<=n&&sumb+b[j]<=suma){
sumb+=b[j];
ans = max(ans,sumb-i-j);
j++;
}
}
return ans;
}
int main(){
daj_mi_malo_vremena
cin >> n;
v.resize(n+1);
w.resize(n+1);
for(ll i = 1;i<=n;i++){
cin >> v[i] >> w[i];
}
sort(v.begin()+1,v.end(),cmp);
sort(w.begin()+1,w.end(),cmp);
ld ans = max(reshi(v,w),reshi(w,v));
cout<<fixed<<setprecision(4);
cout<<ans<<endl;
return 0;
}
/*
4
1.4 3.7
1.2 2
1.6 1.4
1.9 1.5
*/
