#include<bits/stdc++.h>
using namespace std;
typedef long long ll;
typedef pair<int, int> pii;
const int MAX = 2e6 + 15;
const int INF = 0x3f3f3f3f;
const int MOD = 1e9 + 7;
#define pb push_back
#define sz(x) (int) x.size()
#define fr first
#define sc second
#define mp make_pair
#define all(x) x.begin(), x.end()
int n;
long double a[MAX], b[MAX];
int main(){
ios_base::sync_with_stdio(false);
cin.tie(NULL);
cin >> n;
for(int i = 0; i < n; i++){
cin >> a[i] >> b[i];
}
sort(a, a + n, greater<long double>());
sort(b, b + n, greater<long double>());
int pa = 1, pb = 1;
long double sa = a[0], sb = b[0], ans = max((long double) 0, min(sa - 2, sb - 2));
while(pa != n && pb != n){
if(sa < sb){
sa += a[pa];
pa++;
}else{
sb += b[pb];
pb++;
}
ans = max(ans, min(sa - (pa + pb), sb - (pa + pb)));
}
cout << setprecision(4) << fixed << ans << '\n';
return 0;
}
/* stuff you should look for
* int overflow, array bounds
* special cases (n=1?)
* do smth instead of nothing and stay organized
* brute force to find pattern?
* sort?
* graph?
* WRITE STUFF DOWN
* DON'T GET STUCK ON ONE APPROACH
*/
