#include <bits/stdc++.h>
#define mp make_pair
#define pb push_back
#define F first
#define S second
#define all(x) x.begin(),x.end()
#define MAXN 100005
typedef long long ll;
using namespace std;
int n;
double a[MAXN],b[MAXN],prefb[MAXN],prefa[MAXN],res;
int bin(int l,int d,int pa,int sol){
if(l>d)return sol;
int mid=(l+d)/2;
if(prefa[pa]-mid>prefb[mid]-pa)return bin(mid+1,d,pa,mid);
return bin(l,mid-1,pa,sol);
}
int main()///NE MORA DA VUDE POVEZAN
{
scanf("%d", &n);
for(int i=1;i<=n;i++){
scanf("%lf %lf",a+i,b+i);
}
double res=0.0;
sort(a+1,a+1+n,greater<double>());
sort(b+1,b+1+n,greater<double>());
for(int i=1;i<=n;i++){
prefb[i]=prefb[i-1]+b[i]-1.0;
prefa[i]=prefa[i-1]+a[i]-1.0;
}
for(int i=1;i<=n;i++){
int ok=bin(0,n,i,0);
//printf(" %d %d %f %f\n",i,ok,prefa[i],prefb[ok]);
res=max(res,min(prefa[i]-ok,prefb[ok]-i));
if(ok!=n)res=max(res,min(prefa[i]-ok-1,prefb[ok+1]-i));
}
printf("%.4f",res);
return 0;
}
