How do I add the sum of 1st 3 elements of a vector in a for loop?

I'd like for my for loop to end at n-1 as I have other code in the body. I just want to add a line in the for loop to add the sum of the 1st 3 elements underneath the left end points in the for loop and display it outside

t=0:5; a=[32 19.41 11.77 7.14 4.33 2.63];     
n=length(a); a1=a(1:3);
S=0; 
for k=1:n-1;
    S=S+sum(a(k));  % left end-points = upper estimate because a is decreasing
end
disp(S)

I'd like the code to do this with the desired output:

S = 32+ 51.41 + 63.18 = 146.59

[Walter Roberson]

S=S+sum(a(1:k));

[rezheen]

In this case it's:

S=S+sum(a(1:k-2));

Thanks.

Answer 1

[Stephen23]

a = [32,19.41,11.77,7.14,4.33,2.63];
m = 3;
s = sum(cumsum(a(1:m)))

s = 146.5900

or

s = a(1:m)*(m:-1:1).'

s = 146.5900

or

s = dot(a(1:m),m:-1:1)

s = 146.5900

or

s = sum(a(1:m).*(m:-1:1))

s = 146.5900

or

s = sum(a(1:m)*triu(ones(m)))

s = 146.5900

or

s = sum(repelem(a(1:m),m:-1:1))

s = 146.5900

[rezheen]

That also works.