Malloc header contents

So in most implementations malloc stores an header before the allocated memory to keep track of the allocated memory size (so that it can do free and recalloc). What are the header contents?

I wrote a naive code to find it but it doesn't make any sense

int * ptr;
ptr = malloc(12*sizeof(int));
printf("Header = %d\n",*(ptr-1));

It returns

Header = 57

What is happening here?