Post and Pre increment

Question

The output of the following code

int k=10;
int g=5;
int s=g+k++;
cout<<s;

is 15

but the output of the following code

int k = 10;
int s= ++k + k++;
cout << s;

is coming as 23. What is the correct explanation? this was doubt number #1

doubt number #2 is

int m = 10;
int n = 10;
int a = 1 + (++m) + (++(++m))++;
int b = (++n) + (++(++n))++;

how is the value of both a and b the same which is 27