C++ x doesnt loop increament

I have a task from my teacher,like : x^2 + y^3 = z

x filled only with odd y filled only with even

#include <stdio.h>
#include <string>
#include <iostream>

using namespace std;
int x,y,z;

int main(){
   for (x=1;x<=20;x++){
      if ((x%2==1)&&(y%2==0)){
         for (y=1;y<=20;y++){
            if ((x%2==1)&&(y%2==0)){
               z = (x*x) + (y*y*y);
               cout << "x^2 + y^3 =" <<z <<"\n";
            }
         }
      }
   }
}

I try to make my own code like above ,but the only one loop is Y , x stand still with 1.

I want to make x to be looping too. What should i do?

PS. Im sorry with my bad english :D