Preventing letter/symbol input

[mose1234]

Hi, basically trying to stop letters and symbols being entered into my main function. For the menu section I used a buffer yet I try this with my function and its not working. Any help would be great appreciated and it is for the code below.

#include <iostream>
#include <string>
#include <math.h>
#include <windows.h>

 using namespace std;

 struct Point3D
 {
	float x;
	float y;
	float z;
 } V1, V2;

	double CalculateDistance(Point3D * V1, Point3D * V2);
	double CalculateLenth(Point3D* * V1, Point3D* * V2);

int main(void)

{
	string buffer;
	char choice;    // variables
	double distance;
	double length;

	do
	{
		do
		{
			system("CLS");
			cout << "Welcome to the Distance Calulator for 3D points" << endl; //main menu
			cout << "1. Go" << endl; //commence distance calculating
			cout << "2. Quit" << endl; //quit the application

			cin >> buffer;
		} while(buffer.length() > 1); //limiting the user to enter 1 numerical character to being the application

		choice = buffer[0];	

		if (choice == '1')

		{
			cout << "Enter Vertex 1's X co-ordinate: "; 
			cin >> V1.x;
			cout << "Enter Vertex 1's Y co-ordinate: ";      // Vertex 1's co-ordinates
			cin >> V1.y;
			cout << "Enter Vertex 1's Z co-ordinate: ";
			cin >> V1.z;
			cout << "Enter Vertex 2's X co-ordinate: ";
			cin >> V2.x;
			cout << "Enter Vertex 2's Y co-ordinate: ";		 // Vertes 2's co-ordinates
			cin >> V2.y;
			cout << "Enter Vertex 2's Z co-ordinate: ";
			cin >> V2.z;

			distance = CalculateDistance(&V1, &V2); 

			cout << "Distance between V1 and V2 = " << distance << endl; // outputting the total distance
			cout << " " << endl;;
			cout << "Do you want to calculate another distance ?" << endl; // option to carrying on the application
			cout << "Press '1' to go back to the main menu, or '2' to quit." << endl; 
			cin >> choice;

		}

	} while(choice != '2'); //the start of the sequence if the user chooses to end the application

	cout << "Goodbye";
	Sleep(500); // makes the computer 'wait' for 500 milli-seconds

	return 0; // end the application
}

	double CalculateDistance(Point3D * V1, Point3D * V2)
{
	double distance;
	distance = sqrt(pow(V1 -> x - V2 -> x, 2) +
						pow(V1 -> y - V2 -> y, 2) +  // distance calculator for the first function
						pow(V1 -> z - V2 -> z, 2));
	return distance;
}