#include <bits/stdc++.h>
#define taskname "surebet"
using namespace std;
typedef long long ll;
typedef pair<int,int> ii;
typedef unsigned long long ull;
#define fi first
#define se second
#define pb push_back
#define pf push_front
#define es erase
#define is insert
#define all(x) x.begin(), x.end()
#define EL printf("\n")
#define sz(A) (int) A.size()
#define FOR(i,l,r) for (int i=l;i<=r;i++)
#define FOD(i,r,l) for (int i=r;i>=l;i--)
#define fillchar(a,x) memset(a, x, sizeof (a))
#define faster ios_base::sync_with_stdio(false); cin.tie(NULL); cout.tie(NULL);
//#pragma GCC optimize("O3")
#pragma GCC optimize("Ofast")
//#pragma GCC optimize("Os")
//#pragma GCC target ("avx2")
#pragma GCC target("sse,sse2,sse3,ssse3,sse4,popcnt,abm,mmx,avx,avx2,fma")
#pragma GCC optimize("unroll-loops")
priority_queue <int,vector<int>,greater<int> > pq;
int n ;
const int maxn = 1e5+13;
long double a[maxn], b[maxn];
bool cmp (int x , int y)
{
return x > y;
}
void readInput()
{
cin>>n;
for(int i=1 ; i<=n ; ++i) cin>>a[i]>>b[i];
sort(a+1, a+1+n );
sort(b+1, b+1+n );
bool ok = false;
long double ans = 0;
int cnt = 0;
long double suma = 0;
long double sumb = 0;
int k1 = n;
int k2 = n;
while(true)
{
if(k1 == 0 && k2 == 0) break;
if(k1 == 0)
{
sumb += b[k2];
k2--;
cnt++;
}
else if(k2 == 0)
{
suma += a[k1];
k1--;
cnt++;
}
else if(suma<sumb )
{
suma += a[k1];
k1--;
cnt++;
}
else if( suma > sumb)
{
sumb += b[k2];
k2--;
cnt++;
}
else if(suma == sumb)
{
if(k2 == 0 || a[k1] > b[k2])
{
suma += a[k1];
k1--;
cnt++;
}
else if(k1 == 0 || a[k1] <= b[k2])
{
sumb += b[k2];
k2--;
cnt++;
}
}
long double res = min(suma - cnt , sumb - cnt);
ans = max(ans , res);
}
cout<<fixed << setprecision(4) <<ans;
}
void init()
{
}
void solve()
{
}
void print()
{
}
int main()
{
faster
// freopen(taskname".INP","r",stdin);
// freopen(taskname".OUT","w",stdout);
readInput();
solve();
print();
return 0;
}
/* test
*/
