#include <bits/stdc++.h>
using namespace std;
#define int long long
const int N=1e6+5;
int n; double res;
double a[N],b[N];
void ql(int i,double x,double y,int used)
{
if (i>2*n)
{
res=max(res,min(x,y)-used);
return;
}
ql(i+1,x,y,used);
if (i<=n) ql(i+1,x+a[i],y,used+1);
else ql(i+1,x,y+b[i-n],used+1);
}
signed main()
{
ios_base::sync_with_stdio(NULL);
cin.tie(NULL); cout.tie(NULL);
cin >> n;
for (int i=1;i<=n;i++)
cin >> a[i] >> b[i];
res=0;
ql(1,0,0,0);
cout << setprecision(4) << fixed << res;
}
