Thread: Sum of octals found in input

I really want to understand character I/O in C but I feel like my understanding is faulty about certain concepts. I wanna write a program that gives me the sum of all base-8 numbers found in input, but the sum is never incremented.

Code:

/* Write a function that reads data from standard input until EOF is found and returns the sum of the base-8 numbers found into the input.
 We consider a number in base-8 a sequence that starts with 0 and is followed by digits from range [0-7].
 The numbers must be separated from other words by at least one whitespace.
  All the numbers and the final sum are guaranteed to fit in "unsigned" data type. */


  #include <stdio.h>


  int main(){
      int current, last;
      unsigned sum=0;
      int n = 0;
    printf("Enter text:\n");
     current = getchar();
     while(current != '\n'){
            putchar(current);
            last = current;
            current = getchar();
        if(last == '0' && isdigit(current)){
            last = current;
            current = getchar();
            while(isdigit(current)){
                n = current- '0';
                sum = sum+n;
            }
        }
     }


     printf("\nThe sum is %d", sum);
     return 0;
  }

Input: John has 033 apples and 02 strawberries
Output:
John has 033 apples and 02 strawberries
The sum is 0
Please bear in mind that, although I am familiar with strings, I shouldn't really use them for this solution (not that I'd know how to anyway). 'current' and 'last' are meant to define the current char and the last char from the input. I don't really understand how can I 'construct' my number from the characters, since I don't have to add them digit by digit but as whole numbers. Could you help me?