#include <bits/stdc++.h>
using namespace std;
#define int int64_t
#define ld long double
#define pii array<int,2>
vector<ld> prefSum(vector<ld> &a) {
vector<ld> p(a.size());
p[0] = a[0];
for (int i=1;i<(int)a.size();i++) p[i]=p[i-1]+a[i];
return p;
}
signed main() {
int n;cin>>n;
vector<ld> a(n), b(n);
for (int i=0;i<n;i++) {
cin>>a[i]>>b[i];
}
sort(a.begin(), a.end(), greater<ld>());
sort(b.begin(), b.end(), greater<ld>());
vector<ld> pa = prefSum(a), pb = prefSum(b);
ld l = 0, r = 1e9;
while (abs(l - r) >= 0.000001) {
ld m = (l+r) / 2.0;
bool pos = false;
for (int i=0;i<n;i++) {
ld wn = (pa[i] - (ld)(i+1)) - m;
if (wn <= 0.0) continue;
wn = floor(wn);
int k = (int)wn;
if (k <= 0) continue;
k = min(k, n);
ld pw = pb[k-1] - (ld)(i+1+k);
if (pw >= m) {
pos = true;
break;
}
}
if (pos) {
l = m;
}
else {
r = m;
}
}
printf("%.4lf\n", (double)l);
}
