#include <bits/stdc++.h>
using namespace std;
int const maxn = 1e5+1;
vector<double> A,B;
int main()
{
ios_base::sync_with_stdio(false);cin.tie(0);cout.tie(0);
int n;
cin>>n;
for (int i=0;i<n;i++){
double a,b;
cin>>a>>b;
A.push_back(a);B.push_back(b);
}
sort(A.begin(),A.end());sort(B.begin(),B.end());reverse(A.begin(),A.end());reverse(B.begin(),B.end());
int p1 = 0,p2 = 0;
double odg = 0.0,an1 = 0.0,an2 = 0.0;
while (p1<n || p2<n)
{
if (p1 == n || an2<=an1){
an2 += B[p2];
p2++;
}
else if (p2 == n || an1<=an2){
an1 += A[p1];
p1++;
}
double k = p1+p2;
odg = max(odg, min(an1,an2)-k);
//cout<<"p1 = "<<p1<<" p2 = "<<p2<<endl;
}
printf("%.4lf",(double)odg);
return 0;
}
