C++ :

#include<iostream>
#include<cstdio>
using namespace std;
int main()
{
	int x,t=0;
	double ans=0;
	while (cin>>x && x!=-1)
		if (x%2==0)
		{
			ans+=x;
			t++;
		}
	printf("%.2lf\n",ans/t);
	return 0;
}

Pascal :

program p2690(input,output);
var
  n,s,t:longint;
  a:real;
begin
  n:=0;
  s:=0;
  t:=0;
  while n<>-1 do
    begin
      read(n);
      if (n mod 2=0) then
        begin
          s:=s+n;
          t:=t+1;
        end;
    end;
  a:=s/t;
  writeln(a:0:2);
end.