First, you should specify tha values that k, p, n, i get. For example: k = 1:1:4 which means that k can take tha values 1, 2, 3, 4. Then, you can use 'if' statement to separate the two situations. For the sum, you can use the 'sum' function: http://uk.mathworks.com/help/matlab/ref/sum.html
Hope it helps a bit.
Antonios
