[Rhino24]

Obtain a series of positive numbers from the keyboard, and determine and display the sum of the numbers. Assume that the user types the sentinel value -1 to indicate “end of data entry”

[TheRobatron]

This may not be proper C syntax, because I really only know C++, but it should help.
It's very simple really...

int main() {
int num, total;
while (num != -1) {
cout << "Please enter your number. ";
cin >> num;
total += num;
}
total++ // Because -1 was added to it on the last loop
cout << "\nThe total of your numbers is " << total;
}

I hope that helps even though it's not in C