#include <stdio.h>

int scanM (); // scans the size of the array
void scanArr (int M, float *Arr); // scans in the array
int calc (float *Arr, int M); // calculates P

int main (void)
{
	int M;
	M = scanM ();

	float Arr[M];
	scanArr (M, Arr);

	double P;
	P = calc (Arr, M);
	printf ("P is equal to: %f", P);

	return 0;
}

int scanM ()
{
	int M;

	printf ("Enter the size of the array (0-51): ");
	do
	{
		scanf ("%d", &M);
	}
	while (M < 1 || M > 50);

	return M;
}

void scanArr (int M, float *Arr)
{
	int i;

	for (i = 0; i < M; i++)
	{
		printf ("Enter the %d member of the array: ", i);
		scanf ("%f", &Arr[i]);
	}

	/*for (i = 0; i < M; i++)
	{
		printf ("Element nr %d is: %.2f\n", i, Arr[i]);
	}*/
}

int calc (float *Arr, int M)
{
	int i = 0;
	double P;

	P = Arr[i] * (Arr[i+1] + Arr[i+2]);
	i++;

	return P;
}