function assign to variable in C -

why following code return error when variable declared global.

int add(int x, int y) {     return x+y; }   int ab = add(10, 20); int main(void) {      printf("%d", ab); }

but if call this:

int add(int x, int y) {     return x+y; }   int main(void) {     int ab = add(10, 20);  // variable declare inside main     printf("%d", ab); }

then executes without error.

initializers global variables must constant, can't arbitrary expression.

KHS