C++ :

#include<cstdio>
using namespace std;
int main()
{
	int s,i,n;
	scanf("%d",&n);
	s=0;
	for(i=1;i<=n;i++)
	{
		if(i%3==0)
		{
			s=s+i;
		}
	}
	printf("%d\n",s);
	return 0; 
}

Pascal :

Program jjisgay;
 var
  n,i,s:longint;
begin
  read(n);
  for i:= 1 to n do
    if (i mod 3=0) then s:=s+i;
  write(s);
end.