Random results when using pthread

I wrote a simple program using pthread but my results are random....

#define NTHREADS 2
pthread_mutex_t mutex1 = PTHREAD_MUTEX_INITIALIZER;


void *add(void* numbers){
  pthread_mutex_lock( &mutex1 );

  int *n = (int*) numbers;
  float sum;
  for(int i = 0; i < 5; i++){
     sum = sum + n[i] +5;
  }
  cout << sum/5<<endl;

  pthread_mutex_unlock( &mutex1 );

}


void *substract(void* numbers){
  pthread_mutex_lock( &mutex1 );

  int *n = (int*) numbers;
  float sum;
  for(int i = 0; i < 5; i++){
   sum = sum + n[i] -10;
  }
  cout << sum/5<<endl;

  pthread_mutex_unlock( &mutex1 );
}
main(){
  pthread_t thread_id[NTHREADS];
  int i, j;

  int *numbers = new int[5];
  numbers[0] = 34; numbers[1] = 2; numbers[2]= 77; numbers[3] = 40; numbers[4] = 12;
  pthread_create( &thread_id[0], NULL, add, (void*) numbers);
  pthread_create( &thread_id[1], NULL, substract, (void*) numbers );

  pthread_join( thread_id[0], NULL);
  pthread_join( thread_id[1], NULL);

  exit(EXIT_SUCCESS);
}

The output of the program is random....Sometimes it got

 -2.42477e+26
  23

Sometimes it got only one strange number such as

 235.69118e+13
 (empty space)

I have also tried only to use one thread, but the result is still random. For example, I only used thread to calculate "add", the result is sometimes 38, which is correct, but sometimes is a very strange number.

Where I did wrong? Thank you .