#include <bits/stdc++.h>
using namespace std;
constexpr int NMAX = 1e5 + 5;
int N;
double A[NMAX];
double B[NMAX];
int main()
{
ios_base::sync_with_stdio(false);
cin.tie(nullptr);
cin >> N;
for (int i = 1; i <= N; ++ i ) {
cin >> A[i] >> B[i];
}
sort(A+1, A+N+1);
sort(B+1, B+N+1);
for (int i = N - 1; i >= 1; -- i )
B[i] += B[i+1];
double answer = 0;
double sumA = 0;
for (int i = N+1; i >= 1; -- i ) {
sumA += A[i];
int paidA = (N - i + 1);
int st = 1, dr = N;
int pos = N + 1;
while (st <= dr) {
int mij = (st + dr) / 2;
double curr_case = min(sumA - paidA - (N - mij + 1), B[mij] - paidA - (N - mij + 1));
double ant_case = min(sumA - paidA - (N - (mij+1) + 1), B[mij+1] - paidA - (N - (mij+1) + 1));
if (curr_case >= ant_case) {
pos = mij;
dr = mij - 1;
}
else st = mij + 1;
}
double sumB = B[pos];
int paidB = N - pos + 1;
answer = max(answer, min(sumA - paidA - paidB, sumB - paidA - paidB));
}
cout << setprecision(4) << fixed << answer << '\n';
return 0;
}
