#include <bits/stdc++.h>
using namespace std;
#define int long long
#define pll pair<int, int>
#define mp make_pair
#define pb push_back
#define f first
#define s second
#define ld long double
signed main(){
int n;cin>>n;
ld a[n], b[n];
for(int i=0;i<n;i++)cin>>a[i]>>b[i];
sort(a, a+n, greater<ld>());
sort(b, b+n, greater<ld>());
ld pa[n], pb[n];
pa[0]=a[0], pb[0]=b[0];
for(int i=1;i<n;i++){
pa[i]=pa[i-1]+a[i];
pb[i]=pb[i-1]+b[i];
}
ld ans=0;
for(int i=0;i<n;i++){
int x;
x=lower_bound(pb, pb+n, pa[i])-pb;
if(x!=n){
ans=max(ans, pa[i]-x-i-2);
}
x=lower_bound(pa, pa+n, pb[i])-pa;
if(x!=n){
ans=max(ans, pb[i]-x-i-2);
}
}
cout<<fixed<<setprecision(4)<<ans;
}
