#include <bits/stdc++.h>
using namespace std;
#define int long long
signed main()
{
ios_base::sync_with_stdio(false);
cin.tie(0);
int n;
cin >> n;
vector<double> a(n + 1);
vector<double> b(n + 1);
for (int i = 1; i <= n; i++) cin >> a[i] >> b[i];
sort(b.rbegin(), b.rend());
sort(a.rbegin(), a.rend());
//cout << '\n';
vector<double> pa(n + 1, 0);
vector<double> pb(n + 1, 0);
pa[0] = a[0];
pb[0] = b[0];
for (int i = 1; i < n; i++) {
pa[i] = pa[i - 1] + a[i];
pb[i] = pb[i - 1] + b[i];
}
// cout << '\n';
// for (int i = 0; i < n; i++) cout << pa[i] << ' ';
// cout << '\n';
// for (int i = 0; i < n; i++) cout << pb[i] << ' ';
// cout << '\n';
double ans = 0;
for (int i = 0; i < n; i++) {
double c = i + 2;
for (int j = 0; j < n; j++) {
//cout << ct1 << ' ' << ct2 << '\n';
ans = max(ans, min(pa[i] - j, pb[j] - j) - c);
}
}
printf("%.4lf", ans);
return 0;
}
/*
max (min(a[i] - j, b[j] - j) - i))
a[i] < a[i + 1] - 1
l = a[i] - i
k = b[j] - j
0<=i,j<n
min(a[i] - j, b[j] - j)
b[j] < b[j + 1]
j
*/
