How to reduce the fitness value?

function Y = objfun(X)

% Y = fitness value

% X = variable vector (1*n)

% R = 1.994*r^-0.32*f^2.34*v^-0.01*d^0.18

% where r=[0.2:0.8], f=[0.2:0.6], v=[39.27:117.81], d=[0.2:0.8]

r=X(1,1); % variable 1

f=X(1,2); % variable 2

v=X(1,3); % variable 3

d=X(1,4); % variable 4

Y = 1.994*r^-0.32*f^2.34*v^-0.01*d^0.18;