Cannot multiply 2 variables with cvx package

As I'm writing a test code for solving an applied problem with cvx, I got an error:

Disciplined convex programming error:
    Invalid quadratic form(s): not a square.

Here's my code:

cvx_begin 
    variables p q s t P C R;
    maximize(P); 
    subject to
        P = R-C;
        C = 400000+195*s+225*t
        R = p(1)*s(1)+q(1)*t(1)
        p = 339-0.01*s-0.003*t;
        q = 399-0.004*s-0.01*t;
        s >= 0;
        t >= 0;
cvx_end

Could anyone help? I'm not used in cvx using