32 lines
663 B
C
32 lines
663 B
C
// Try to use recursion to solve this problem.
|
|
#include <stdio.h>
|
|
|
|
// function prototypes.
|
|
|
|
unsigned long long int sum(unsigned long long int n);
|
|
|
|
int main(void) {
|
|
|
|
// Initialize variables and get input from user.
|
|
unsigned long long int num;
|
|
scanf("%lld", &num);
|
|
|
|
// Calculate the sum of the numbers. (Can be optimized.)
|
|
unsigned long long int num1 = 0;
|
|
for (int i = 1; i <= num; i++) {
|
|
num1 += sum(i);
|
|
}
|
|
|
|
printf("%lld\n", num1);
|
|
return 0;
|
|
}
|
|
|
|
unsigned long long int sum(unsigned long long int n) {
|
|
if (n == 1 || n == 2) { // Case 1: n is 1 or 2
|
|
return 0;
|
|
} else { // Case 2: n is not
|
|
return n * (n - 1) * (n - 2) + sum(n - 1);
|
|
}
|
|
}
|
|
|