#include <bits/stdc++.h>
using namespace std;
typedef long double ld;
int main(){
cin.tie(0);
ios_base::sync_with_stdio(0);
int n;
cin>>n;
vector<ld> vals1(n);
vector<ld> vals2(n);
vector<ld> pref1(n+1);
vector<ld> pref2(n+1);
vector<ld> prefind1(n+1);
vector<ld> prefind2(n+1);
for (int i = 0; i<n; i++){
cin>>vals1[i]>>vals2[i];
}
sort(vals1.rbegin(),vals1.rend());
sort(vals2.rbegin(),vals2.rend());
for (int i = 1; i<=n; i++){
pref1[i]=pref1[i-1]+vals1[i-1]-1;
prefind1[i]=pref1[i]+i;
pref2[i]=pref2[i-1]+vals2[i-1]-1;
prefind2[i]=pref2[i]+i;
}
ld maxval = 0;
for (int i = 0; i<=n; i++){
//smallest x such that prefind1[i]<prefind2[x]
//cout<<"x "<<i<<'\n';
int lb = lower_bound(prefind2.begin(),prefind2.end(),prefind1[i])-prefind2.begin();
//cout<<lb<<'\n';
if (lb!=n+1){maxval=max(maxval,pref1[i]-lb);}
int ub = lb-1;
//cout<<ub<<'\n';
if (ub!=-1){maxval=max(maxval,pref2[ub]-i);}
}
cout<<fixed<<setprecision(4)<<maxval<<'\n';
}
