#include <bits/stdc++.h>
#define pb push_back
#define F first
#define S second
using namespace std;
typedef long long ll;
typedef string str;
const ll Mod = 1e9 + 7;
const int Maxn = 2e5 + 100;
double a[Maxn], b[Maxn], ans;
int n;
int main(){
ios::sync_with_stdio(0); cin.tie(0); cout.tie(0);
cin >> n;
for(int i = 0; i < n; i++) cin >> a[i] >> b[i];
sort(a, a + n);
sort(b, b + n);
reverse(a, a + n);
reverse(b, b + n);
b[n] = 1e9;
int p = 0;
double c = 0, s1 = 0, s2 = 0;
for(int i = 0; i < n; i++){
s1 += a[i]; c++;
ans = max(ans, min(s1, s2) - c);
while(s2 < s1){
s2 += b[p ++]; c += 1;
ans = max(ans, min(s1, s2) - c);
}
}
cout << fixed << setprecision(4) << ans << '\n';
return 0;
}
/*
4
1.4 3.7
1.2 2
1.6 1.4
1.9 1.5
*/
