Help with validation rule

Hi I am writing some code but need a validation in one part

Code:

for(int i=0; i < size; i++)
  {
  system("cls");
cout <<"\nPlease enter the Cars registration: " << (i+1) << ": ";
cin >> autos[i].carreg;
cout <<"\nPlease enter the make of Car: ";
cin >> autos[i].carmake;
 cout <<"\nPlease enter the Model of Car: ";
cin >> autos[i].carmodel;
 cout <<"\nPlease enter the Cars milage: ";
cin >> autos[i].carmile;
cout <<"\nPlease enter the Cars age: ";
cin >> autos[i].carage;
cout<<"\n";
  }

I need it so that you cannot enter a minus value for car age or milage, im sure i know this but my mind has gone totally blank.
Thanks in advance for any help.
snooki

Without knowing the types of the variables (eg are they unsigned) the obvious response is to do a run time check on the value.

Code:

autos[i].carage = -1;
while (autos[i].carage < 0)
{
    cout <<"\nPlease enter the Cars age: ";
    cin >> autos[i].carage;

    if (autos[i].carage < 0) complain_bitterly();
}