Perform the sum using data from the API in flutter

permission to ask, I want to ask how do I do the addition using data in the API, I’ve been having trouble for days and haven’t solved it either
I’ve got the reference but still confused too

this is the json data from the API.

{
            "id_transkrip_nilai": "53a6c0c7-9271-4b1a-8b2e-bf942cb0871c",
            "id_mk": "b91b6b58-3c22-45e2-8aca-f0fe507d229f",
            "kode_mk": null,
            "nm_mk": "KPAM-VII (Kapita Selekta)",
            "sks": 1,
            "smt": 7,
            "nilai_akhir_uts": "100",
            "nilai_huruf_uts": "A",
            "nilai_indeks_uts": "4",
            "nilai_akhir_uas": "90",
            "nilai_akhir": "93.33",
            "nilai_huruf_akhir": "A",
            "nilai_indeks_akhir": "4",
            "status_nilai_akhir": 3,
            "status_nilai_uts": 3,
            "updated_by": "Raey"
        }

I want to add the value of nilai_indeks_uts + nilai_indeks_akhir and I have made the model before.

this is a simple example if the data is already set and comes from locale

example

var firstNumber = 4;    
var secondNumber = 13;  
var sum = firstNumber + secondNumber;  
print(sum);

/*    
Output :    
 17  
*/

how do i do it if the data is coming from API. please enlighten. Thanks.

class NilaiMahasiswa {
  String? status;
  String? code;
  List<Data>? data;

  NilaiMahasiswa({this.status, this.code, this.data});

  NilaiMahasiswa.fromJson(Map<String, dynamic> json) {
    status = json['status'];
    code = json['code'];
    if (json['data'] != null) {
      data = <Data>[];
      json['data'].forEach((v) {
        data!.add(Data.fromJson(v));
      });
    }
  }

  Map<String, dynamic> toJson() {
    final Map<String, dynamic> data = <String, dynamic>{};
    data['status'] = status;
    data['code'] = code;
    if (this.data != null) {
      data['data'] = this.data!.map((v) => v.toJson()).toList();
    }
    return data;
  }
}

class Data {
  String? idTranskripNilai;
  String? idMk;
  String? kodeMk;
  String? nmMk;
  int? sks;
  int? smt;
  String? nilaiAkhirUts;
  String? nilaiHurufUts;
  String? nilaiIndeksUts;
  String? nilaiAkhirUas;
  String? nilaiAkhir;
  String? nilaiHurufAkhir;
  String? nilaiIndeksAkhir;
  int? statusNilaiAkhir;
  int? statusNilaiUts;
  String? updatedBy;

  Data(
      {this.idTranskripNilai,
      this.idMk,
      this.kodeMk,
      this.nmMk,
      this.sks,
      this.smt,
      this.nilaiAkhirUts,
      this.nilaiHurufUts,
      this.nilaiIndeksUts,
      this.nilaiAkhirUas,
      this.nilaiAkhir,
      this.nilaiHurufAkhir,
      this.nilaiIndeksAkhir,
      this.statusNilaiAkhir,
      this.statusNilaiUts,
      this.updatedBy});

  Data.fromJson(Map<String, dynamic> json) {
    idTranskripNilai = json['id_transkrip_nilai'];
    idMk = json['id_mk'];
    kodeMk = json['kode_mk'];
    nmMk = json['nm_mk'];
    sks = json['sks'];
    smt = json['smt'];
    nilaiAkhirUts = json['nilai_akhir_uts'];
    nilaiHurufUts = json['nilai_huruf_uts'];
    nilaiIndeksUts = json['nilai_indeks_uts'];
    nilaiAkhirUas = json['nilai_akhir_uas'];
    nilaiAkhir = json['nilai_akhir'];
    nilaiHurufAkhir = json['nilai_huruf_akhir'];
    nilaiIndeksAkhir = json['nilai_indeks_akhir'];
    statusNilaiAkhir = json['status_nilai_akhir'];
    statusNilaiUts = json['status_nilai_uts'];
    updatedBy = json['updated_by'];
  }

  Map<String, dynamic> toJson() {
    final Map<String, dynamic> data = <String, dynamic>{};
    data['id_transkrip_nilai'] = this.idTranskripNilai;
    data['id_mk'] = this.idMk;
    data['kode_mk'] = this.kodeMk;
    data['nm_mk'] = this.nmMk;
    data['sks'] = this.sks;
    data['smt'] = this.smt;
    data['nilai_akhir_uts'] = this.nilaiAkhirUts;
    data['nilai_huruf_uts'] = this.nilaiHurufUts;
    data['nilai_indeks_uts'] = this.nilaiIndeksUts;
    data['nilai_akhir_uas'] = this.nilaiAkhirUas;
    data['nilai_akhir'] = this.nilaiAkhir;
    data['nilai_huruf_akhir'] = this.nilaiHurufAkhir;
    data['nilai_indeks_akhir'] = this.nilaiIndeksAkhir;
    data['status_nilai_akhir'] = this.statusNilaiAkhir;
    data['status_nilai_uts'] = this.statusNilaiUts;
    data['updated_by'] = this.updatedBy;
    return data;
  }
}