C++: Length and Height Conversion Program

#include <iostream>
using namespace std;

int meter_mill (int m)
{
  int mill;
  mill=m*1000;
  return (mill);
}
int meter_cent (int a)
{
  int cent;
  cent=a*100;
  return (cent);
}
double meter_kil (double a )
{
  double kil;
  kil=a*.001;
  return (kil);
}
double meter_inc (double a)
{
  double inc;
  inc=a*39.37008;
  return (inc);
}
 double meter_feet (double a)
{
  double feet;
  feet=a*3.28084;
  return (feet);
}
double meter_yard (double a)
{
  double yard;
  yard=a*1.09361;
  return (yard);
}
double meter_mile (double a)
{
  double mile;
  mile=a*6.2;
  return (mile);
}
double meter_naut_mile (double a)
{
  double naut_mile;
  naut_mile=a*5.4;
  return (naut_mile);
}
int main ()
{

  char choice;
  double mm,cm,km,in,ft,yrd,m,nm;
  double n;
  cout<<"Choose a Unit of Measurement: \na-Meter-Millimeter \nb-Meter-Centimeter\nc-Meter-Kilometer\nd-Meter-Inches\ne-Meter-Feet\nf-Meter-Yard\nj-Meter-Mile\nk-Meter-Nautical Mile"<<endl;
  cin>>choice;
  switch (choice)
  {
  case 'a':
    cout<<"Enter Number of Meter: ";
    cin>>n;
    mm = meter_mill (n);
    cout << "The height in milimeter is "<<mm<<endl;
    break;
    case 'b':
    cout<<"Enter Number of Meter: ";
    cin>>n;
    cm = meter_cent (n);
    cout << "The height in centimeter is "<<cm<<endl;
    break;
    case 'c':
    cout<<"Enter Number of Meter: ";
    cin>>n;
    km = meter_kil (n);
    cout << "The height in Kilometer is "<<km<<endl;
    break;
    case 'd':
    cout<<"Enter Number of Meter: ";
    cin>>n;
    in = meter_inc (n);
    cout << "The height in Inches is "<<in<<endl;
    break;
    case 'e':
    cout<<"Enter Number of Meter: ";
    cin>>n;
    ft = meter_feet (n);
    cout << "The height in Feet is "<<ft<<endl;
    break;
    case 'f':
    cout<<"Enter Number of Meter: ";
    cin>>n;
    yrd = meter_yard (n);
    cout << "The height in Yard is "<<yrd<<endl;
    break;
    case 'j':
    cout<<"Enter Number of Meter: ";
    cin>>n;
    m = meter_mile (n);
    cout << "The height in Miles is "<<m<<endl;
    break;
    case 'k':
    cout<<"Enter Number of Meter: ";
    cin>>n;
    nm = meter_naut_mile (n);
    cout << "The height in Nautical Miles  is "<<nm<<endl;
    break;
    default:
    cout<<"Out of coverage"<<endl;
    }
  return 0;
}