Initializer element is not constant in C

I'm coming from javascript/php/python and probably I'm missing something, here is the code:

const int a = 50;
const int c = 100;
const int d = 100;
int endX = c + a;
int endY = d;
int startX, startY, b;

I get

ex1.4.c:6: error: initializer element is not constant
ex1.4.c:7: error: initializer element is not constant

Does someone have an explanation?