#include<bits/stdc++.h>
#define fi first
#define se second
using namespace std;
const int N = 2e6+7;
int n;
double a[N], b[N], prefixSumA[N], prefixSumB[N], res = 0;
int main()
{
cin>>n;
for(int i=1;i<=n;i++) cin>>a[i]>>b[i];
sort(a+1,a+1+n,greater<double>());
sort(b+1,b+1+n,greater<double>());
for(int i=1;i<=n;i++) {
prefixSumA[i] = prefixSumA[i-1] + a[i];
prefixSumB[i] = prefixSumB[i-1] + b[i];
}
int j = 1;
for(int i=1;i<=n;i++) {
while(j < n && prefixSumA[j + 1] <= prefixSumB[i]) j++;
if(prefixSumB[i] < prefixSumA[j]) res = max(res, double(prefixSumB[i] - i - j)); else
{
res = max(res, double(prefixSumA[j] - i - j));
if(j < n) res = max(res, double(prefixSumB[i] - i - j - 1));
}
}
cout<<fixed<<setprecision(4)<<res;
return 0;
}
