#include <bits/stdc++.h>
using namespace std;
int main() {
int n;
cin >> n;
vector <double> a(n), b(n);
for (int i = 0; i < n; i++)
cin >> a[i] >> b[i];
sort(a.begin(), a.end(), greater <double> ());
sort(b.begin(), b.end(), greater <double> ());
double ans = 0;
int l = 0, r = 0;
double suma = 0, sumb = 0, tot = 0;
while (l < n || r < n) {
if ((suma < sumb && l < n) || r >= n) {
suma += a[l];
tot++;
l++;
} else {
sumb += b[r];
tot++;
r++;
}
ans = max(ans, min(suma, sumb) - tot);
}
cout << fixed << setprecision(4) << ans << '\n';
}
