#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef double ld;
typedef pair < int, int > PII;
#define F first
#define S second
#define mkp make_pair
#define eb emplace_back
#define sz(a) (int)a.size()
#define all(a) a.begin(), a.end()
#define y1 kekek
#define left(v) v << 1
#define right(v) v << 1 | 1
#define forn(x, a, b) for (int x = a; x <= b; ++x)
#define for1(x, a, b) for (int x = a; x >= b; --x)
const ll ool = 1e18 + 9;
const int N = 2e5 + 6, oo = 1e9 + 9, base = 1e9 + 7;
int n;
ld a[N], b[N], pref[N];
pair < ld, ld > mymin(pair < ld, ld > a, pair < ld, ld > b) {
if (min(a.F, a.S) < min(b.F, b.S)) return b;
return a;
}
int main() {
ios_base :: sync_with_stdio(0);
cin.tie(0);
#ifdef krauch
freopen("input.txt", "r", stdin);
#endif
cin >> n;
forn(i, 1, n) {
cin >> a[i] >> b[i];
}
sort(a + 1, a + n + 1);
reverse(a + 1, a + n + 1);
sort(b + 1, b + n + 1);
reverse(b + 1, b + n + 1);
forn(i, 1, n) {
pref[i] += pref[i - 1] + b[i] - 1;
}
ld sum = 0;
pair < ld, ld > ans = mkp(0, 0);
forn(i, 1, n) {
sum += a[i] - 1;
int l = 0, r = n;
while (l < r) {
int mid = (l + r) >> 1;
if (pref[mid] - i >= sum - mid) r = mid;
else l = mid + 1;
}
forn(j, max(l - 2, 0), min(l + 1, n)) {
ans = mymin(ans, mkp(pref[j] - i, sum - j));
}
}
cout << fixed << setprecision(4) << min(ans.F, ans.S) << "\n";
return 0;
}
