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#include <fstream>
using namespace std;
ifstream fin("fact.in");
ofstream fout("fact.out");
long long p, n, st, dr, m, cm, prod, nr;
int main()
{
fin>>p;
if( p < 0) fout<<-1;
else if( p == 0) fout<<1;
else{
st=1;
dr=5*p;
while( st <= dr)
{
m = st + (dr - st)/2;
cm=m;
nr=0;
while(cm)
{
nr+=(cm/5);
cm/=5;
}
if( p > nr) st = m + 1;
else dr= m - 1;
}
fout<<st;
}
return 0;
}