#include<bits/stdc++.h>
using namespace std;
double a[100010]={0};
double b[100010]={0};

bool cmp(double x,double y)
{
	return x>y;
}
int main()
{
	freopen("wager.in","r",stdin);
	freopen("wager.out","w",stdout);
	int n,a1,b1,i,j;
	double ansa,ansb,ans;
	cin>>n;
	for(i=1;i<=n;i++)
	{
		cin>>a[i]>>b[i];;
	}
	sort(a+1,a+n+1,cmp);
	sort(b+1,b+n+1,cmp);
	ansa=a[1]-2;ansb=b[1]-2;a1=2;b1=2;ans=min(ansa,ansb);
	while(true)
	{
		if(ansa<ansb)
		{
			if(a1<=n)
			{
				ansa=ansa+a[a1]-1;
				a1++;
				ansb--;
			}
			else
			{
				break;
			}
		}
		else
		{
			if(b1<=n)
			{
				ansb=ansb+b[b1]-1;
				b1++;
				ansa--;
			}
			else
			{
				break;
			}
		}
		if(ans<min(ansa,ansb))
		{
			ans=min(ansa,ansb);
		}
	}
	if(ans<=0)
	{
		cout<<"0.0000";
	}
	else
	{
		cout<<fixed<<setprecision(4)<<ans;
	}
	return 0;
}