Please provide explanation

Prove explanation main.c #include void func(); int x = 1; int y; int z = 3; int main(){func(); printf("%d ", x + y + z); return 0;} func.c int x; static int y = 2; int z; void func() {x = x + y; y = 4 + z; z = x + y;} The code above is compiled using "gcc main.c func.c". Output of the above program: 20 13 6 3 Undefined behaviour