#include<bits/stdc++.h>
using namespace std;
#define int long long
#define inf 5e18
#define nl '\n'
inline void solve(){
int n;
cin>>n;
double a[n], b[n];
for(int i=0; i<n; i++) cin>>a[i]>>b[i];
sort(a, a+n, greater<double>());
sort(b, b+n, greater<double>());
int j = 0;
double sa = 0, sb = 0, ans = 0;
for(int i=0; i<n; i++){
sa += a[i] - 1.0;
sb -= 1.0;
while(j < n and sb+b[j] < sa){
sb += b[j] - 1.0;
sa -= 1.0;
j++;
}
ans = max(ans, sb);
if(j < n) ans = max(ans, sa - 1.0);
}
cout<<fixed<<setprecision(4)<<ans;
}
signed main(){
ios_base::sync_with_stdio(0);
cin.tie(NULL);cout.tie(NULL);
int t = 1;
//cin>>t;
while(t--) solve();
return 0;
}
