#include<bits/stdc++.h>
using namespace std;
int n,k,j;
double a[100005],b[100005],sum1,s1,sum2,ans;
int main(){
cin>>n;
for(k=1;k<=n;k++){
cin>>a[k]>>b[k];
}
sort(a+1,a+n+1);
sort(b+1,b+n+1);
j=n;
for(k=n;k>=1;k--){
sum1+=a[k]-1;
sum2--;
while(sum2<sum1 && j>=1){
sum1-=1; sum2+=b[j]-1;
ans=max(ans,min(sum1,sum2));
j--;
}
} sum1=sum2=s1=0; j=n;
for(k=n;k>=1;k--){
sum1+=b[k]-1;
sum2--; //cout<<sum2<<" "<<endl;
while(sum2<sum1 && j>=1){
sum1-=1; sum2+=a[j]-1;
ans=max(ans,min(sum1,sum2));
j--;
}
}
printf("%.4lf",(double)ans);
}
