#include <stdio.h>

main()
{
	long s, s0;
	int i, j;

	s = 0L;
	for (i = 1; i <= 20; i++) {
		s0 = 1L;
		for (j = 1; j <= i; j++)
			s0 *= j;
		s += s0;
	}
	printf("%ld\n", s);
}