#include <bits/stdc++.h>
using namespace std;
#define int long long
signed 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.rbegin(), a.rend());
sort(b.rbegin(), b.rend());
double ans = 0;
int l = 0, r = 0;
double sa = 0, sb = 0, s = 0;
while (l < n || r < n) {
if ((sa < sb and l < n) || r >= n) {
sa += a[l];
s++;
l++;
}
else {
sb += b[r];
s++;
r++;
}
ans = max(ans, min(sa, sb) - s);
}
cout << fixed << setprecision(9) << ans;
return 0;
}
