S.E.M.F.物理C代码

它说错误：标识符“a5”是未定义的,我知道这意味着什么,但如果我输入偶数甚至奇数或奇数奇怪的A Z值,我将如何从选择语句中获取a5.

#include <iostream>
#include <cstdlib>
#include <math.h>

using namespace std;

int main()
{
    int A,Z;

    // Main body of Semi-Empirical Mass Formula
    cout <<"Enter the mass number A: ";
    cin >> A;
    cout <<"n";
    cout <<"Enter the atomic number Z: ";
    cin >> Z;
    cout <<"n";

    // Constants from formula,units in MeV(millions of electron volts)
    double a1 = 15.67;          
    double a2 = 17.23;
    double a3 = 0.75;
    double a4 = 93.2;

    if(Z % 2 == 0 && (A - Z) % 2 == 0)

        double a5 = 12.0;

    else if(Z % 2 != 0 && (A - Z) % 2 != 0)

        double a5 = -12.0;

    else

        double a5 = 0;


    // Formula for to compute the binding energy
    double B =a1 * A - a2 * pow( A,2/3) - a3 * (pow(Z,2) / pow(A,1/3)) - a4 * (pow(A - 2 * Z,2) / A) + (a5 / pow(A,1/2));

    // Formula for to compute the binding energy per nucleon
    double B_E = B / A;


    return 0;

}