Can you please explain this program that includes a recursive function and how 21 is obtained if the input value is n=6??

#include static int MyFunction (int n) Enter a non-negative integer value n: 6 MyFunction) returned the value retval-21 if (n-0) return(0); return (n+MyFunction (n-1)) / end of MyFunction)/ int main(int argc, char *kargv) int n; int retval; printf("Enter a non-negative scanf("%d", &n); integer value n: "); if ((n65535)) fprintf(stderr,"%s : return (-1); n out of range [0,65535]. ", argv [0]); retval-MyFunction (n); printf("MyFunction() returned the value retval-sd ", retval); return(0); / end of main)