#include<bits/stdc++.h>
typedef long long ll;
#define pb push_back
#define fr first
#define sc second
#define endl '\n'
using namespace std;
int n;
double a[100023],b[100023];
int main(){
ios_base::sync_with_stdio(false);cin.tie(NULL);
cin>>n;
for(int i=1;i<=n;i++){
cin>>a[i]>>b[i];
}
sort(a+1,a+n+1,[&](const double&x,const double&y){return x>y;});
sort(b+1,b+n+1,[&](const double&x,const double&y){return x>y;});
int l=1,r=1;
double sum1=0,sum2=0;
double ans=0;
for(int i=1;i<=2*n;i++){
if(l>n||(r<=n&&sum1>sum2)){
sum2+=b[r++];
}
else if(r>n||(l<=n&&sum1<=sum2)){
sum1+=a[l++];
}
ans=max(ans,min(sum1,sum2)-double(i));
}
printf("%.4lf",ans);
}
