How to converted this to For loop?

How to convert this to for loop? Im doing it several times to convert it but its hard to get the logic of it.

int total, gradecounter, grade;
double average;

total = 0;
gradecounter = 0;

cout<<"Enter grade, -1 to end";
cin>> grade;

while (grade !=-1) 
   {
    cout<<"Enter grade, -1 to end:";
    cin>>grade;
    total = total + grade;
    gradecounter = gradecounter + 1;
   }
if (gradecounter !=0) 
   {
    average = total/gradecounter;
    cout<<"Class Average is "<<average<<endl;
    getche();
    return 0;
   }

getche();
return 0;