/// 1n the name of GOD
#include <bits/stdc++.h>
using namespace std;
#define ko exit(0);
#define nl '\n'
#define ios ios_base :: sync_with_stdio(0), cin.tie(0), cout.tie(0);
#define sz(s) (int)s.size()
#define all(s) s.begin(), s.end()
#define oll(s, n, c) (s) + (c), s + (n) + c
#define pb push_back
#define bb begin()
#define ee end()
#define ff first
#define ss second
#define pii pair<int, int>
//#define for(i, l, r) for(int (i) = (l); i <= (r); ++i)
typedef long long ll;
typedef double db;
const int N = 1e5 + 777;
const int M = INT_MAX;
int n;
double a[N], b[N];
double c, d;
int main(){
ios
cin >> n;
for(int i = 1; i <= n; ++i){
cin >> a[i] >> b[i];
}
sort(oll(a, n, 1));
sort(oll(b, n, 1));
reverse(oll(a, n, 1));
reverse(oll(b, n, 1));
int id = 1;
int di = 1;
double ans = 0;
c += a[1];
d += b[1];
ans = max(ans, min(c - (double)2, d - (double)2));
while(id <= n && di <= n){
while(d > c){
id++;
c += a[id];
ans = max(ans, min(c - (double)(id + di), d - (double)(di + id)));
}
while(d < c){
di++;
d += b[di];
ans = max(ans, min(c - (double)(id + di), d - (double)(di + id)));
}
if(d == c){
id++;
di++;
c += a[id];
d += b[di];
ans = max(ans, min(c - (double)(id + di), d - (double)(di + id)));
}
}
cout << fixed << setprecision(4) << (double)ans;
return 0;
}
