Which usage of overloaded subtraction '-' operator is invalid

For a struct Complex that represents a complex number:

struct Complex {
 int real;
 int imaginary;
};

the non-member subtraction operator (-) is implemented as:

const Complex operator - (const Complex& c1, const Complex& c2) {
 Complex result = {
   c1.real - c2.real,
   c1.imaginary - c2.imaginary
  };
 return result;
}

Given 3 instances of Complex:

Complex c1 = { 10, 23 };
Complex c2 = { 7, 25 };
Complex c3 = { 15, 30 };

Which one of followings usage of subtraction operator ('-') is not valid (will not compile):