C++ loop help

i have a littel problem with my program
i need to make progrem that will
enter numbers positive (min 4 numbers) and to finish the progrem in the end need to writh -1

o.k up to now i menage to do this:
#include <iostream.h>
void main ()
{
int sum=0, counter, num, n=0;
cin >> num;
do
{
counter = n;
do
{
sum = sum + num;
counter ++;
cin >> num;
}
while (counter <=n+3 && num != -1) ;
cout <<"the sum is:"<<sum<<endl;
n= n+1;
}
while (counter <=n+3);

}