#include <bits/stdc++.h>
using namespace std;
int main()
{
long long int count=0,n,i=0,m;
cin>>m;
n = m;
while(1)
{
if(n-n/10<10)
break;
count += n/10;
n -= n/10;
i++;
}
i++;
i=i/2;
m/=2;
cout<<ceil(float(m)/float(i));
//cout<<i<<" "<<count;
return 0;
}
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