#include <bits/stdc++.h>
using namespace std;
#define endl '\n'
#define ll long long
#define all(x) (x).begin(), (x).end()
int main() {
ios_base::sync_with_stdio(false);
cin.tie(NULL);
int n;
cin >> n;
vector<double> a(n), b(n);
for (int i = 0; i < n; i++) cin >> a[i] >> b[i];
sort(all(a));
reverse(all(a));
sort(all(b));
reverse(all(b));
for (int i = 1; i < n; i++) a[i] += a[i - 1];
for (int i = 1; i < n; i++) b[i] += b[i - 1];
int r = 0;
double ans = 0;
for (int i = 0; i < n; i++) {
while (b[r] < a[i]) {
ans = max(ans, b[r] - (i + r + 2));
r++;
}
if (r < n) ans = max(ans, a[i] - (i + r + 2));
ans = max(ans, b[r - 1] - (i + r + 1));
}
cout << fixed << setprecision(4) << ans;
}
