Log program in C++

#include<iostream.h>
#include<conio.h>
#include<math.h>
int main()
{
          int a;
          float r;
          cout<<"Enter a number :";
          cin>>a;
          r=log(a);
        
          cout<<"the log of  "<<a<<"  is :"<<r;
          getch();
          return 0;
          }