Boolean in c Program

#include
#include
#include

typedef int Bool;
const Bool TRUE = 1;
const Bool FALSE = 0;

void readLetter(char&);
int computeCorrespondingDigit(char);
Bool doOneLetter();

void main ()
{
  Bool flag = FALSE;

  while (!flag)
{
    flag = doOneLetter();
  }
}

int exitLetter (char c)
{
  return ((c == 'Q') || (c == 'Z'));
}

Bool doOneLetter()
{
  char l;
  int i;

  readLetter(l);
  if (exitLetter(l))
{
    cout << "Quit." << endl;
    return(TRUE);
  }
  Else
 {
    i = computeCorrespondingDigit(l);
    cout << "The letter " << l << " corresponds to " << i << " on the telephone"<< endl;
    return(FALSE);
  }
}

int computeCorrespondingDigit (char l)
{
  switch (l)
{
  case 'A' :
  case 'B' :
  case 'C' :
    return(2);
  case 'D' :
  case 'E' :
  case 'F' :
    return(3);
  case 'G' :
  case 'H' :
  case 'I' :
    return(4);
  case 'J' :
  case 'K' :
  case 'L' :
    return(5);
  case 'M' :
  case 'N' :
  case 'O' :
    return(6);
  case 'P' :
  case 'R' :
  case 'S' :
    return(7);
  case 'T' :
  case 'U' :
  case 'V' :
    return(8);
  case 'W' :
  case 'X' :
  case 'Y' :
    return(9);
  }
}

void readLetter(char& c)
{
  cout << "Enter a letter: ";
  cin >> c;
  while (cin && !isalpha(c))
{
    cout << "You have not entered a letter. Try again: ";
    cin >> c;
  }
  if (!cin)
{
    cout << "Unrecoverable error... Exiting" << endl;
    exit(-1);
  }
  c = toupper(c);
}