#include <bits/stdc++.h>
using namespace std;
int n;
long double a[100005], b[100005], sum[100005], ans = 0;
void solve(){
sort(a + 1, a + n + 1);
sort(b + 1, b + n + 1);
for(int i = n; i >= 1; i--) sum[i] = sum[i + 1] + b[i];
long double suma = 0;
for(int i = n; i >= 1; i--){
suma += a[i];
int l = 0, r = n + 1;
while(r - l > 1){
int mid = (l + r) / 2;
if(sum[mid] > suma) l = mid;
else r = mid;
}
int num = (n - i + 1) + (n - r + 1);
ans = max(ans, min(suma, sum[r]) - num);
ans = max(ans, min(suma, sum[r] + b[l]) - num - 1);
}
cout << fixed << setprecision(4) << ans;
}
signed main(){
ios_base::sync_with_stdio(NULL); cin.tie(nullptr); cout.tie(nullptr);
cin >> n;
for(int i = 1; i <= n; i++) cin >> a[i] >> b[i];
solve();
}
