string trouble in c

Question

The question is when I input a string with "sum" in begin and then compute the following number.

Input:

sum 10 20 

Output:

30

but my code is wrong,the output is 33 (the processing is 11+22=33) I think that the second while loop has wrong ,but I don't know how to revise.

I need any master help.

#include<stdio.h>

int main(){

    char a[100];

    while (gets(a))
    {
        if (a[0] == 's'&&a[1] == 'u'&&a[2] == 'm')
        {
            int i;
            int sum = 0;

            for (i = 2; a[i]; i++){
                if (a[i] == ' '){
                    i++;
                    int num = 0;
                    while (1){
                        num += num * 10 + (a[i] - '0');
                        i++;
                        if (a[i] == ' ' || a[i]=='\0') break;
                    }
                    sum += num;
                    i--;
                }
            }

            printf("%d", sum);
        }

    }
    return 0;
}

Answer 1

num += num * 10 + (a[i] - '0');

should be

num = num * 10 + (a[i] - '0');