#include <iostream>
#include <algorithm>
#include <vector>
#include <cmath>
using namespace std;
int main() {
ios_base::sync_with_stdio(0);
cin.tie(0);
long long n;
cin >> n;
vector<double> a(n), b(n);
for (long long i = 0; i < n; i++) {
cin >> a[i] >> b[i];
a[i] -= 1;
b[i] -= 1;
}
sort(a.begin(), a.end());
reverse(a.begin(), a.end());
sort(b.begin(), b.end());
reverse(b.begin(), b.end());
double ans = 0;
vector<double> prefb(n);
prefb[0] = b[0];
for (long long i = 1; i < n; i++) {
prefb[i] = prefb[i - 1] + b[i];
}
double sum = 0;
for (long long i = 0; i < n; i++) {
sum += a[i];
double ansi = sum + i + 1;
long long l = 0, r = n - 1;
while (l != r) {
long long mid = (l + r + 1) / 2;
double ansmid = prefb[mid] + mid + 1;
if (ansmid < ansi) {
l = mid;
}
else {
r = mid - 1;
}
}
ans = max(ans, min(sum - l - 1, prefb[l] - i - 1));
if (l != n - 1) {
l++;
}
ans = max(ans, min(sum - l - 1, prefb[l] - i - 1));
}
printf("%.4lf", (double)ans);
}
