Could anyone please explain how these expressions are evaluated?

#include <bits/stdc++.h>
using namespace std;

int main()
{
  int a = 1;
  a = a++ + a++;
  cout<<a<<endl;
  a = 1;
  a = ++a + ++a;
  cout<<a<<endl;
  a = 1;
  a = a++ + ++a;
  cout<<a<<endl;
  a = 1;
  a = ++a + a++;
  cout<<a<<endl;
  
  return 0;
}

OUTPUT: ~~~~~ 3 6 4 5 ~~~~~