void readdata(){
    //void readPi0(char const *file){

    TFile *f = new TFile("small.root");

    TTree* Experiment = (TTree*)f->Get("Experiment");
    Int_t run_num;
    Experiment->SetBranchAddress("TGlobalTankHeaderChunk.data_.RUN_NUM", &run_num);
    //Int_t previous_run_num=0;
    //Int_t count=0;

    //ofstream myfile;
    //myfile.open("test2.txt");
    Long64_t nentries = Experiment->GetEntries();

    for ( Long64_t  jentry = 0 ; jentry < nentries; ++jentry){
        //Long64_t ientry = Experiment->LoadTree(jentry);

        Experiment->GetEntry(jentry);
        std::cout<<run_num<<std::endl;

    }

    return;
}