#include <bits/stdc++.h>
#define int long long
#define INF 1000000000000000
#define lchild(i) (i*2 + 1)
#define rchild(i) (i*2 + 2)
#define mid(l, u) ((l+u)/2)
#define x(p) p.first
#define y(p) p.second
#define MOD 998244353
using namespace std;
bool cmp(pair<pair<double, double>, int> a, pair<pair<double, double>, int> b){
if(a.first.first < b.first.first) return true;
if(b.first.first < a.first.first) return false;
if(a.first.second > b.first.second) return true;
return false;
}
signed main(){
int n;
cin>>n;
double a[2][n];
for(int i = 0;i<n;i++) cin>>a[0][i]>>a[1][i];
sort(a[0], a[0]+n);
sort(a[1], a[1]+n);
if(a[0][n-1]<=2 && a[1][n-1]<=2){
cout<<"0\n";
return 0;
}
int ind[] = {n-1, n-1};
vector<pair<pair<double, double>, int>> pq;
double ans = 0;
int inv = 0;
pq.push_back({{0, a[0][n-1]}, 0});
pq.push_back({{0, a[1][n-1]}, 1});
sort(pq.begin(), pq.end(), cmp);
while(ind[pq[0].second] >= 0){
pair<pair<double, double>, int> curr = pq[0];
curr.first.first += a[curr.second][ind[curr.second]--];
inv++;
if(ind[curr.second]>=0) curr.first.second = a[curr.second][ind[curr.second]];
else curr.first.second = -INF;
pq[0] = curr;
sort(pq.begin(), pq.end(), cmp);
ans = max(ans, pq[0].first.first - inv);
}
printf("%.4lf",(double)ans);
}
