Modulo

std::array<int, 5> modulos{11, 107, 1013, 19211, 81727};

for (auto i = 0u; i < arr.size(); ++i) {
  sum += arr[i] % modulos[i * 5 / arr.size()];
}

for (auto i = 0u; i < arr.size(); ++i) {
  switch (i * 5 / arr.size()) {
    case 0:
      sum += arr[i] % 11;
      break;
    case 1:
      sum += arr[i] % 107;
      break;
    case 2:
      sum += arr[i] % 1013;
      break;
    case 3:
      sum += arr[i] % 19211;
      break;
    case 4:
      sum += arr[i] % 81727;
      break;
  }
}