Randperm without number 1

I am using randperm to select positions that will be changed. How can I change my code so that the number 1 (position 1) is not included?

N = 50;
N20 = .2*N;
r = randperm(N,N20); %positions that are about to be changed
for y=r
    S20a(:,y)=2*round(rand(1)) - 1;
end

Answer 1

[Stephen23]

r = 1+randperm(N-1,N20);