import java.io.*;
class poisson
{
	public static void main(String arg[ ]) throws IOException
	{

		BufferedReader in = new BufferedReader(new
			InputStreamReader(System.in));	
	
		System.out.println("Opciones : ");
		System.out.println( );
		System.out.println("1 : Evaluar Poisson");
		System.out.println("2 : Sumatoria de variables Poisson");

		System.out.println( );
		System.out.print("Ingrese opcion : ");

		int opcion = Integer.parseInt(in.readLine( ));

		System.out.println();

		double lamda;
		switch(opcion)
		{
			case 1 : 
				
				int x;
				System.out.print("Ingrese lamda : "); 	
				lamda = Double.parseDouble(in.readLine( ));
                
				System.out.print("Ingrese x : ");
				x = Integer.parseInt(in.readLine( ));
                
				System.out.println();
				System.out.print("Resultado : ");
				System.out.println(fpoisson(lamda,x));
				break;

			case 2 :
				
				int xi,xf;
				System.out.print("Ingrese lamda : "); 	
				lamda = Double.parseDouble(in.readLine( ));
                
				System.out.print("Ingrese x inicial : ");
				xi = Integer.parseInt(in.readLine( )); 
				System.out.print("Ingrese x final : "); 
				xf = Integer.parseInt(in.readLine( )); 
				System.out.println(); 
				System.out.print("Resultado : "); 
				System.out.println(fpoissonAcum(lamda,xi,xf));  
				break;
		} 
	}
        
	public static double fpoisson(double lamda , int x ) 
	{ 
		double p = Math.pow(lamda,x) / factorial(x);
		return p * Math.exp(-lamda);
	}
                
	public static double fpoissonAcum(double lamda, int xi, int xf) 
	{ 
		double s = 0.0; 
		for(int x = xi; x <= xf; x++)
		{
			s += fpoisson(lamda,x); 
		}
    
		return s; 
	}  
    
	public static long  factorial(int n)
	{
		long f = 1; 
		if(n > 1)  
		{	
			while (n > 1)   
			{
				f *= n--; 
			}    
		}
		return f;
	}

}