#include<bits/stdc++.h>
#include<fstream>
using namespace std;
//ifstream fin("FEEDING.INP");
//ofstream fout("FEEDING.OUT");
#define sz(a) (int)a.size()
#define ll long long
#define pb push_back
#define forr(i, a, b) for(int i = a; i < b; i++)
#define dorr(i, a, b) for(int i = a; i >= b; i--)
#define ld long double
#define vt vector
#include<fstream>
#define fi first
#define se second
#define pll pair<ll, ll>
#define pii pair<int, int>
const int base = 74;
const ll mod = 1e9 + 7, inf = 1e9, mxv = 1005, mxn = 2e5 + 5;
mt19937 rng(chrono::steady_clock::now().time_since_epoch().count());
int n;
ld a[mxn + 1], b[mxn + 1], pa[mxn + 1], pb[mxn + 1];
ld val(int x, int y){
return(min(pa[x], pb[y]) - (x + y));
}
signed main()
{
ios_base::sync_with_stdio(0); cin.tie(0); cout.tie(0);
cin >> n;
for(int i = 1; i <= n; i++){
cin >> a[i] >> b[i];
}
ld ans = 0;
sort(a + 1, a + n + 1, greater<ld>()); sort(b + 1, b + n + 1, greater<ld>());
for(int i = 1; i <= n; i++){
pa[i] = pa[i - 1] + a[i]; pb[i] = pb[i - 1] + b[i];
}
int l = 1, r = 1;
ans = max(ans, min(pa[1], pb[1]) - (1 + 1));
while(l != n|| r != n){
if(l == n)r++;
else if(r == n)l++;
else if(pa[l] < pb[r])l++;
else r++;
ans = max(ans, min(pa[l], pb[r]) - (l + r));
//cout << l << " " << r << " ";
}
cout << fixed << setprecision(4) << ans;
return(0);
}
