#include<bits/stdc++.h>
#define ll long long
#define db double
using namespace std;
const int N=100005;
double A[N+3],B[N+3];
int n;
int main() {
ios_base::sync_with_stdio(0);
cin.tie(0);
cin>>n;
for(int i=1;i<=n;i++){
cin>>A[i]>>B[i];
}
sort(A+1,A+n+1,greater<double>());
sort(B+1,B+n+1,greater<double>());
double ans=0.0,sa=0.0,sb=0.0;
int pa=1,pb=1;
for(int i=1;i<=2*n;i++){
db ma=sa;
if(pa<=n)ma=sa+A[pa]-i;
db mb=sb;
if(pb<=n)mb=sb+B[pb]-i;
db ca=min(ma,sb-i);
db cb=min(mb,sa-i);
if(ca>=cb){
sa+=A[pa];
++pa;
ans=max(ans,ca);
}
else{
sb+=B[pb];
++pb;
ans=max(ans,cb);
}
}
cout<<fixed<<setprecision(4)<<ans;
return 0;
}
/*
4
1.4 3.7
1.2 2
1.6 1.4
1.9 1.5
*/
