#include<bits/stdc++.h>
using namespace std;
#define F first
#define S second
#define ll long long
#define FORi(i,a,b) for(ll i=a;i<(ll)b;i++)
#define FOR(i,n) FORi(i,0,n)
#define FOA(v,a) for(auto v : a)
#define MID ((l+r)/2)
int n;
vector<double> a, b;
int main(){
cin>>n;
a.resize(n);
b.resize(n);
FOR(i,n) cin>>a[i]>>b[i];
sort(a.begin(), a.end(), [](double a, double b){return a>b;});
sort(b.begin(), b.end(), [](double a, double b){return a>b;});
double ans=0;
double sa=0;
double sb[n];
sb[0] = b[0];
FORi(i,1,n) sb[i] = sb[i-1] + b[i];
FOR(i,n){
sa+=a[i];
int nxt = lower_bound(sb, sb+n, sa)-sb;
if(nxt!=n+1)
ans = max(ans, min(sb[nxt]-i-1-nxt-1, sa-nxt-1-i-1));
if(nxt!=0){
nxt--;
ans = max(ans, min(sb[nxt]-i-1-nxt-1, sa-nxt-1-i-1));
}
}
cout<<fixed<<setprecision(4)<<ans<<endl;
}
