Some task

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

unsigned long long int i,n,a,s;
float s1;

int main() {

  scanf("%lli %lli\n",&n,&a);

  if (a != 1) {

  for (i=1;i<=n;i++) {

  s = s + i*pow(a,i);

  }
  printf("%lli\n",s);

  }
  else
  {
  s1 = n*(n+1)/2;
  s = s1;
  printf("%lli\n",s);
  }

  return 0;
}