// Author: Teoman Ata Korkmaz
#include <bits/stdc++.h>
#define int int64_t
using namespace std;
///////////////////////////////////////////////////////////
int n;
vector<double> a,b;
signed main(void){
cin>>n;
a.resize(n);
b.resize(n);
for(int i=0;i<n;i++)cin>>a[i]>>b[i];
sort(a.rbegin(),a.rend());
sort(b.rbegin(),b.rend());
for(int i=1;i<n;i++)a[i]+=a[i-1];
for(int i=1;i<n;i++)b[i]+=b[i-1];
// cerr<<fixed<<setprecision(4);
// cerr<<"a:";for(auto i:a)cerr<<" "<<i;cerr<<endl;
// cerr<<"b:";for(auto i:b)cerr<<" "<<i;cerr<<endl;
double ans=0;
for(int i=0;i<n;i++){
for(int j=0;j<n;j++){
ans=max(ans,min(a[i],b[j])-double(i+j+2));
// cerr<<"i,j,ans: "<<i<<" "<<j<<" "<<ans<<endl;
}
}
cout<<fixed<<setprecision(4)<<ans<<endl;
}
/*
* min(a[i],b[j])-i-j
*
* insert(x)
* add(x)
* max()
*/
