#include <iostream>
#include <vector>
#include <cmath>
#include <algorithm>
using namespace std;
double MAX(double a, double b) {
if (a > b) {
return a;
}
return b;
}
double MIN(double a, double b) {
if (a < b) {
return a;
}
return b;
}
int main() {
int n; cin >> n; double profit = 0;
vector<double> a(n); vector<double> b(n);
for (int i = 0; i < n; i++) {
cin >> a[i]; cin >> b[i];
}
sort(a.begin(), a.end()); reverse(a.begin(), a.end()); sort(b.begin(), b.end()); reverse(b.begin(), b.end());
vector<double> pa(n); vector<double> pb(n);
pa[0] = a[0]; pb[0] = b[0];
for (int i = 1; i < n; i++) {
pa[i] = pa[i - 1] + a[i]; pb[i] = pb[i - 1] + b[i];
}
vector<int> x(n); vector<int> y(n);
int j = 0;
for (int i = 0; i < n; i++) {
while (j < n && pa[i] > pb[min(n-1, j)]) {
j++;
}
x[i] = j;
//cout << "x[" << i << "] = " << x[i] << endl;
}
j = 0;
for (int i = 0; i < n; i++) {
while (j < n && pb[i] > pa[min(n - 1, j)]) {
j++;
}
y[i] = j;
}
for (int i = 0; i < n; i++) {
if (x[i] <= n - 1) {
profit = MAX(profit, pa[i] - i - x[i] - 2);
}
if (y[i] <= n - 1) {
profit = MAX(profit, pb[i] - i - y[i] - 2);
}
}
printf("%.4lf", (double)profit);
}
