#include <iostream>
#include <algorithm>
#include <iomanip>
using namespace std;
using ld = double;
const int N = 1e6 + 123;
ld a[N], b[N];
int main() {
ios_base::sync_with_stdio(0), cin.tie(0);
int n;
cin >> n;
for (int i = 1 ; i <= n ; ++ i) cin >> a[i] >> b[i];
sort(a + 1, a + 1 + n);
sort(b + 1, b + 1 + n);
reverse(a + 1, a + 1 + n);
reverse(b + 1, b + 1 + n);
for (int i = 1 ; i <= n ; ++ i) a[i] += a[i - 1], b[i] += b[i - 1];
int l = 1, r = 1;
ld ans = max(0.0, min(a[1], b[1]) - 2);
while (l < n || r < n) {
if (l == n || (r < n && min(a[l], b[r + 1]) >= min(a[l + 1], b[r]))) {
++r;
} else {
++l;
}
ld cur = min(a[l], b[r]) - (l + r);
ans = max(ans, cur);
}
cout << fixed << setprecision(4);
cout << ans;
}
