//author: Ahmet Alp Orakci
#include <bits/stdc++.h>
using namespace std;
using i64 = long long;
#define ONLINE_JUDGE
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);
sort(b, b + n);
double ans = 0, calc1 = 0, calc2 = 0;
int ptr1 = n -1, ptr2 = n -1, al = 0;
while(ptr1 >= 0 || ptr2 >= 0) {
if(calc1 <= calc2) {
if(ptr1 >= 0) {
calc1 += a[ptr1];
ptr1--;
} else {
calc2 += b[ptr2];
ptr2--;
}
} else {
if(ptr2 >= 0) {
calc2 += b[ptr2];
ptr2--;
} else {
calc1 += a[ptr1];
ptr1--;
}
}
al++;
ans = max(ans, min(calc1, calc2) - al);
}
printf("%.4lf", double(ans));
return;
}
signed main() {
#ifndef ONLINE_JUDGE
freopen(".in", "r", stdin);
freopen(".out", "w", stdout);
#endif
int t = 1; //cin >> t;
for(int i = 1; i <= t; i++) {
solve();
}
return 0;
}
