#include <bits/stdc++.h>
using namespace std;
int main(){
ios::sync_with_stdio(false);
cin.tie(NULL);
int n;
cin>>n;
vector<double>a(n),b(n);
for(int i=0;i<n;i++)cin>>a[i]>>b[i];
sort(a.begin(),a.end(),greater<double>());
sort(b.begin(),b.end(),greater<double>());
vector<double>A(n+1),B(n+1);
for(int i=1;i<=n;i++){
A[i]=A[i-1]+a[i-1];
B[i]=B[i-1]+b[i-1];
}
double r=0;
for(int i=0;i<=n;i++){
int l=0,h=n;
while(l<h){
int m=(l+h+1)/2;
if(B[m]>=A[i]) l=m;
else h=m-1;
}
int j=l;
auto u=[&](int k){
if(k<0||k>n) return;
double p=min(A[i],B[k])-(i+k);
if(p>r) r=p;
};
u(j);
u(j+1);
}
if(r<0) r=0;
cout<<fixed<<setprecision(4)<<r<<"\n";
return 0;
}
