#include<bits/stdc++.h>
#define ll long long
#define db double
using namespace std;
const int N=100005;
db A[N+3],B[N+3];
int main(){
ios_base::sync_with_stdio(0);
cin.tie(0);
int n;
cin>>n;
for(int i=1;i<=n;i++){
cin>>A[i]>>B[i];
}
sort(A+1,A+n+1,greater<db>());
sort(B+1,B+n+1,greater<db>());
db sa=0.0,sb=0.0,ans=0.0;
int pa=1,pb=1;
while(pa<=n||pb<=n){
if(pa<=n&&(pb>n||sa<sb)){
sa+=A[pa];
++pa;
}
else if(pb<=n){
sb+=B[pb];
++pb;
}
else break;
db cur=min(sa,sb)-(pa-1+pb-1);
ans=max(ans,cur);
}
cout<<fixed<<setprecision(4)<<ans;
return 0;
}
