#include <bits/stdc++.h>
using namespace std;
#define int long long
#define double long double
void solve(){
int n; cin >> n;
vector<double> a(n + 1), b(n + 1), suma(n + 1), sumb(n + 1);
for (int i = 1; i <= n; i++) cin >> a[i] >> b[i];
sort(a.begin() + 1, a.end(), greater());
sort(b.begin() + 1, b.end(), greater());
for (int i = 1; i <= n; i++) suma[i] = suma[i - 1] + a[i];
for (int i = 1; i <= n; i++) sumb[i] = sumb[i - 1] + b[i];
double res = 0;
for (int i = 0; i <= n; i++){
int j = lower_bound(sumb.begin(), sumb.end(), suma[i]) - sumb.begin();
if(j <= n) res = max(res, suma[i] - (j + i));
if(j >= 1) res = max(res, sumb[j - 1] - (j + i - 1));
}
cout << setprecision(4) << fixed << res;
}
signed main(){
ios_base::sync_with_stdio(false);
cin.tie(0); cout.tie(0);
solve();
}
