#include <bits/stdc++.h>
using namespace std;
int main(){
ios_base::sync_with_stdio(0);cin.tie(0);
vector <double> pie = {}, dru = {};
int n;
cin>>n;
double p1, p2;
for (int i = 0; i<n; i++){
cin>>p1>>p2;
pie.push_back(p1);
dru.push_back(p2);
}
sort(pie.begin(), pie.end(), greater());
sort(dru.begin(), dru.end(), greater());
double w = 0, pl1 = 0, pl2 = 0, ind1 = 0, ind2 = 0;
while(ind1<n && ind2 < n){
while(ind1<n && pl1 <= pl2){
pl1 += pie[ind1]-1;
pl2 -= 1;
w = max(w, min(pl1, pl2));
ind1++;
}
while(ind2<n && pl2 <= pl1){
pl2 += dru[ind2]-1;
pl1-=1;
w = max(w, min(pl1, pl2));
ind2++;
}
}
while(ind1<n && pl1 <= pl2){
pl1 += pie[ind1]-1;
pl2 -= 1;
w = max(w, min(pl1, pl2));
ind1++;
}
cout<<fixed<<setprecision(4)<<w<<"\n";
}
