#include <bits/stdc++.h>
using namespace std;
int main(){
int n;
cin >> n;
long double a[n], b[n];
for (int i = 0; i<n; i++){
cin >> a[i] >> b[i];
}
sort(a,a+n); reverse(a,a+n);
sort(b,b+n); reverse(b,b+n);
long double pfa[n+1], pfb[n+1];
pfa[0] = pfb[0] = 0;
for (int i = 1; i<=n; i++){
pfa[i] = pfa[i-1] + a[i-1];
pfb[i] = pfb[i-1] + b[i-1];
}
int yk = 1;
long double bst = 0;
for (int x = 1; x<=n; x++){
int lo = 0;
int hi = n;
int nxt = lo;
while (lo <= hi){
int mid = (lo+hi)/2;
long double ee = pfa[x]-x-mid;
long double rr = pfb[mid]-mid-x;
if (ee >= rr){
nxt = max(nxt, mid);
lo = mid+1;
}
else{
hi = mid-1;
}
}
long double aa = pfb[nxt]-x-nxt;
bst = max(bst, aa);
if (nxt+1 <= n){
long double bb = pfa[x]-nxt-1-x;
bst = max(bst, bb);
}
}
cout << fixed << setprecision(4) << bst;
return 0;
}
