- (BOOL)isValidCardNumber:(NSString *)number {
    NSInteger *length = number.length;
    int c = [[number substringWithRange:NSMakeRange(15, 1)] intValue];
    int s,k,d = 0;
    if (length < 16 || [[number substringWithRange:NSMakeRange(1, 10)] intValue] == 0 || [[number substringWithRange:NSMakeRange(10, 6)] intValue] == 0) {
        return false;
    }
    for (int i=0; i<16; i++) {
        k = (i % 2 == 0) ? 2 : 1;
        d = [[number substringWithRange:NSMakeRange(i, 1)] intValue] * k;
        s += (d > 9) ? d - 9 : d;
    }
    return ((s % 10) == 0);
}