#include <bits/stdc++.h>
#define pb push_back
#define fi first
#define se second
#define FOR(i,a,b) for(int i = (a) ; i <= (b) ; i++)
#define FORE(i,a,b) for(int i = (a) ; i >= (b) ; i--)
#define par pair<int , int>
#define bit(x,i) ((x>>i)&1)
using namespace std;
const int N = 1 << 20;
const int INF = INT_MAX;
const int mod = 1e9+7;
int MAX(int x, int y){
if(x >= y) return x;
else return y;
}
int MIN(int x, int y){
if(x <= y) return x;
else return y;
}
int n;
double a[N],b[N],ans1,ans2,ans;
double more;
double F[N],G[N];
int m1,m2,l,r;
int main()
{
ios_base::sync_with_stdio(0);
cin.tie(0);
cout.tie(0);
cin >> n;
for(int i = 1 ; i <= n ; i++){
cin >> a[i] >> b[i];
a[i]--;
b[i]--;
}
sort(a+1,a+n+1,[](double u, double v){
return u > v;
});
sort(b+1,b+n+1,[](double u, double v){
return u > v;
});
for(int i = 1 ; i <= n ; i++){
F[i] = F[i-1] + a[i];
G[i] = G[i-1] + b[i];
}
for(int i = 0 ; i <= n ; i++){
l = 0;
r = n;
while(r-l > 2){
m1 = l+(r-l)/3;
if(G[m1]-i <= F[i]-m1){
l = m1;
}
else{
r = m1;
}
}
for(int j = 0 ; j <= 3 ; j++){
if(l+j <= n){
ans = max(ans,min(G[l+j]-i,F[i]-l-j));
}
if(l-j >= 0){
ans = max(ans,min(G[l-j]-i,F[i]-l+j));
}
}
}
cout << setprecision(4) << fixed << ans;
return 0;
}
