#include <stdio.h>
#include <math.h>

int main() {

  double p_factorial = 1;
  int max_itr = 21;

  for (int p = 1; p <= max_itr; p++) 
    p_factorial *= p;

  double sum = 0;
  for (int p = max_itr; p >= 0; p--) {
    sum += 1/p_factorial;
    p_factorial /= p;
  }
  
  printf("sum= %.20f\n", sum);
  printf(" e = %.20f\n", M_E);
  printf("|sum-e|=%e\n", fabs(sum-M_E));
  
  return 0;
}