#include <bits/stdc++.h>
using namespace std;
const int N = 1e5 + 11;
double a[N], b[N];
int main(int argc, char** argv) {
ios::sync_with_stdio(false);
cin.tie(0); cout.tie(0);
int n; cin >> n;
for (int i = 1; i <= n; i++)
cin >> a[i] >> b[i];
sort(a + 1, a + 1 + n, greater<double>());
sort(b + 1, b + 1 + n, greater<double>());
double ans = 0.0;
int pos_a = 1, pos_b = 1, cnt = 0;
double sum_a = 0, sum_b = 0;
while (1) {
++cnt;
if (sum_a < sum_b) {
if (pos_a > n) break;
sum_a += a[pos_a++];
} else if (sum_b < sum_a) {
if (pos_b > n) break;
sum_b += b[pos_b++];
} else {
if (sum_a > n || sum_b > n) break;
sum_a += a[pos_a++];
}
double val = min(sum_a - cnt, sum_b - cnt);
ans = max(ans, val);
}
cout << setprecision(4) << fixed << ans;
}
