#include <bits/stdc++.h>
using namespace std;
#define loop(i,n) for (ll i = 0; i < n; i++)
#define ll long long
#define INF 100005
#define MOD 1000000007
#define pb push_back
#define mp make_pair
typedef pair<ll,ll> pii;
const ll MAXN=1e6;
ll a[MAXN+1];
int main(){
ios::sync_with_stdio(0);
cin.tie(0);
cout.tie(0);
ll n,i,L;
double k;
cin>>n;
double a[n+1],b[n+1];
double Sa[n+1],Sb[n+1];
Sa[0]=0; Sb[0]=0; a[0]=INF; b[0]=INF;
for(i=1;i<=n;i++){
cin>>a[i]>>b[i];
}
sort(a,a+n+1,greater<double>());
sort(b,b+n+1,greater<double>());
for(i=1;i<=n;i++){
Sa[i]=Sa[i-1]+a[i];
Sb[i]=Sb[i-1]+b[i];
}
double ans=0;
for(i=1;i<=n;i++){//cout<<i<<" ";
k=Sa[i];
L=upper_bound(Sb,Sb+n+1,k)-Sb;
//cout<<i<<" "<<k<<" "<<L<<endl;
if(L>1){
k = Sb[L-1] -(L-1)-i;
ans = max(ans,k);//cout<<k<<" ";
}
if(L!=n+1){
k = Sa[i]-i-L;
ans = max(ans,k);//cout<<k<<" ";
}
//cout<<endl;
}
printf("%.4lf\n",(double)ans);
return 0;
}
/*
4
1.4 3.7
1.2 2
1.6 1.4
1.9 1.5
printf("%.4lf %.4lf\n",(double)a[i],(double)b[i]);
*/
