#include<bits/stdc++.h>
#define int long long
#define fi first
#define se second
#define pb push_back
#define ii pair<int, int>
#define sz(v) (int)v.size()
#define all(v) v.begin(), v.end()
using namespace std;
const int N=1e5+5, mod = 1e9+7, inf = 1e18;
int n;
double a[N], b[N];
signed 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]-=1.0; b[i]-=1.0;
}
sort(a+1, a+n+1, greater<double>()); sort(b+1, b+n+1, greater<double>());
for (int i=1; i<=n; i++) {
a[i] += a[i-1]; b[i] += b[i-1];
}
double res = (double) -inf;
for (int na = 0; na<=n; na++) {
int lo = 0, hi = n, ans = -1;
while(lo<=hi) {
int mid = (lo+hi)>>1;
if(a[na] - mid >= b[mid] - na) {
ans = mid;
lo = mid+1;
} else {
hi = mid-1;
}
}
if(ans != -1) res = max(res, b[ans]-na);
if(ans<n) res = max(res, a[na]-ans-1);
}
cout << fixed << setprecision(4) << res;
}
