#include <bits/stdc++.h>
using namespace std;
int main() {
int N; cin >> N;
double A[N+1], B[N+1];
for(int i=1; i<=N; i++) cin >> A[i] >> B[i];
sort(A+1, A+N+1);
sort(B+1, B+N+1);
int pta = N, ptb = N;
double suma = 0, sumb = 0;
double ans = 0;
int cnt = 0;
while(pta || ptb) {
cnt++;
if(suma > sumb) {
if(ptb == 0) break;
sumb += B[ptb--];
}
else if(suma < sumb) {
if(pta == 0) break;
suma += A[pta--];
}
else {
if(pta == 0) sumb += B[ptb--];
else if(ptb == 0) suma += A[pta--];
else if(A[pta] > B[ptb]) suma += A[pta--];
else sumb += B[ptb--];
}
ans = max(ans, min(suma, sumb) - cnt);
}
cout << fixed << setprecision(4) << ans << "\n";
}
