Maman 12 Q4 in C

#include <stdio.h>

// max 10 digists for a 32 bit number plus enter
char NUMBER[13];

// return length of a newline terminated string
int strLen( char *a0 ) {
    int t1 = 0;

    while ( *a0 != 10 ) {
        t1++;
        a0++;
    }

    return t1;
}

int main(void) {
    int s1, s2, s3, t1;
    while ( fgets(NUMBER, 12, stdin) != NULL ) {
        s1 = strLen(NUMBER);
        s2 = 1; // column
        s3 = 0; // result
                t1 = 0; // temp

                s1--; // adjust to be an index
        while ( s1 != -1 ) {
            // check that the char is a digit
                        t1 = *(NUMBER+s1);
            if ( t1 < '0' || t1 > '9' )
                goto error;
                        t1 = t1 - '0';
            s3 += t1 * s2;
            s2 *= 10;
            s1--;
        }
        printf("%d\n", s3);
        if ( s3 >= 100 )
                break;
    }
    return 0;
    error:
    printf("error\n");
    return 1;
}