#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef pair<int,int> pii;
typedef double db;
#define fi first
#define se second
#define mp make_pair
#define fastIO ios::sync_with_stdio(false);cin.tie(0);cout.tie(0);
#define TEST freopen("in.txt","r",stdin);
#define ab(a) ((a < 0) ? (-(a)) : (a))
#define all(a) a.begin(), a.end()
int main(){
fastIO;
int n;
cin >> n;
db a[n];
db b[n];
for(int i = 0;i < n; i++ ){
cin >> a[i] >> b[i];
a[i] -- ;
b[i] -- ;
}
sort(a, a + n);
sort(b, b + n);
reverse(a, a + n);
reverse(b, b + n);
db sum1 = 0;int cnt1 = 0;
db sum2 = 0;int cnt2 = 0;
db ans = 0;
for(int i = 0;i < n;i ++ ){
sum1 += a[i];
cnt1 ++ ;
ans = max(ans, min(sum1-cnt2,sum2-cnt1));
while(sum1-cnt2 > sum2-cnt1 and cnt2 < n){
sum2 += b[cnt2];
cnt2++;
ans = max(ans, min(sum1-cnt2,sum2-cnt1));
}
ans = max(ans, min(sum1-cnt2,sum2-cnt1));
}
cout << fixed << setprecision(4) << ans << "\n";
return 0;
}
