function sim = gaussianKernel(x1, x2, sigma)
    %% RBFKERNEL returns a radial basis function kernel between x1 and x2
    %   sim = gaussianKernel(x1, x2) returns a gaussian kernel between x1 and x2
    %   and returns the value in sim

    % Ensure that x1 and x2 are column vectors
    x1 = x1(:); x2 = x2(:);

    % ====================== YOUR CODE HERE ======================
    % Instructions: Fill in this function to return the similarity between x1
    %               and x2 computed using a Gaussian kernel with bandwidth
    %               sigma
    value = - sum((x1 - x2) .^ 2) / (2 * sigma ^ 2);
    sim = exp(value);
end