#include <bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef unsigned long long ull;
#define MASK(i) (1LL << (i))
#define GETBIT(mask, i) (((mask) >> (i)) & 1)
#define ALL(v) (v).begin(), (v).end()
ll max(ll a, ll b){return (a > b) ? a : b;}
ll min(ll a, ll b){return (a < b) ? a : b;}
ll LASTBIT(ll mask){return (mask) & (-mask);}
int pop_cnt(ll mask){return __builtin_popcountll(mask);}
int ctz(ll mask){return __builtin_ctzll(mask);}
int logOf(ll mask){return 63 - __builtin_clzll(mask);}
mt19937_64 rng(chrono::high_resolution_clock::now().time_since_epoch().count());
// mt19937_64 rng(69);
ll rngesus(ll l, ll r){return l + (ull) rng() % (r - l + 1);}
template <class T1, class T2>
bool maximize(T1 &a, T2 b){
if (a < b) {a = b; return true;}
return false;
}
template <class T1, class T2>
bool minimize(T1 &a, T2 b){
if (a > b) {a = b; return true;}
return false;
}
template <class T>
void printArr(T& container, string separator = " ", string finish = "\n", ostream &out = cout){
for(auto item: container) out << item << separator;
out << finish;
}
template <class T>
void remove_dup(vector<T> &a){
sort(ALL(a));
a.resize(unique(ALL(a)) - a.begin());
}
int main(void){
ios::sync_with_stdio(0); cin.tie(0); cout.tie(0);
int n; cin >> n;
vector<double> a(n), b(n);
for(int i = 0; i<n; ++i) cin >> a[i] >> b[i];
sort(ALL(a), greater<double>()); sort(ALL(b), greater<double>());
vector<double> pref = b;
for(double &i: pref) i -= 1;
for(int i = 1; i<n; ++i) pref[i] += pref[i-1];
cout << fixed << setprecision(4);
double l = 0, r = 1e9;
for(int iteration = 0; iteration < 70; ++iteration){
double x = (l + r) / 2;
bool check = false;
double sum1 = 0;
for(int i = 0; i<n; ++i){
sum1 += a[i] - 1;
if (sum1 >= x){
int bruh = (int) (sum1 - x);
minimize(bruh, n);
if (bruh == 0){
if (x == 0) check = true;
}
else{
if (pref[bruh - 1] >= x + (i + 1)) check = true;
}
}
}
if (check) l = x;
else r = x;
}
cout << l << "\n";
return 0;
}
