14-1.cpp

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#include<iostream.h>
#include<process.h>
void main()
{
  int n;
  unsigned long factor=1;
  cout<<"Enter a number:";
  cin>>n;
  for(int i=n;i>=1;i--)
  factor*=i;
  long ans(n>0?factor:-1);
  if(ans>=0)
    cout<<n<<"!="<<ans;
  else 
  {
    cerr<<"Invalid minus number! \n";
	exit(1);
  }
}