c - What is '-1[p]' when p points to an array (of int) index?

Today I stumbled over a C riddle that got a new surprise for me.

I didn't think that -1[p] in the example below would compile, but it did. In fact, x ends up to be -3.

    int x;
    int array[] = {1, 2, 3};
    int *p = &array[1];
    x = -1[p]

I searched the internet for something like -1[pointer] but couldn't find anything. Okay, it is difficult to enter the correct search query, I admit. Who knows why -1[p] compiles and X becomes -3?