C :

# include <stdio.h>
int main()
{
	int a[10];
	int i=8;
	a[9]=1;
	for(i=8;i>=0;i--)
	{
		a[i]=2*(a[i+1]+1);
	}
	printf("%d\n",a[0]);

	return 0;
}

C++ :

#include<stdio.h>
int main()
{ 
  int day,x1,x2; 
  day=9;
  x2=1;
  while(day>0) 
  {
    x1=(x2+1)*2;
    x2=x1; 
    day--; 
  }
  printf("%d\n",x1); 
}