// Remember...
#include<bits/stdc++.h>
#define F first
#define S second
#define PB push_back
#define sz(s) int((s).size())
#define bit(n,k) (((n)>>(k))&1)
using namespace std;
typedef long long ll;
typedef pair<int,int> pii;
typedef pair<ll,ll> pll;
const int maxn=1e5+10,mod=1e9+7;
const ll inf=1e18;
double arr[2*maxn];
double a[maxn], b[maxn];
int main(){
ios_base::sync_with_stdio(false); cin.tie(0); cout.tie();
int n; cin>>n;
for(int i=0;i<n;i++)
cin>>a[i]>>b[i];
sort(a,a+n), sort(b,b+n);
reverse(a,a+n), reverse(b,b+n);
for(int i=0;i<n;i++)
a[i]+= i==0 ? 0 : a[i-1], b[i]+= i==0 ? 0 : b[i-1], arr[2*i]=a[i], arr[2*i+1]= b[i];
sort(arr,arr+2*n);
int pta=0, ptb=0;
double ans=0;
for(int i=0;i<2*n;i++){
while(pta<n && arr[i]>a[pta]) pta++;
while(ptb<n && arr[i]>b[ptb]) ptb++;
if(arr[i] > min(a[n-1],b[n-1])) break;
ans=max(ans, arr[i] -pta -ptb -2);
}
return cout<<setprecision(4)<<fixed<<ans<<endl,0;
}
// Deathly mistakes:
// * Read the problem carefully.
// * Check maxn.
// * Overflows.
// #pragma GCC optimize("Ofast,no-stack-protector,unroll-loops,fast-math")
// #pragma GCC target("sse,sse2,sse3,ssse3,sse4,popcnt,abm,mmx,avx,tune=native")
