void analysis(){
    // this is to extract the .root files and store them into a vector
    std::vector<string> files;
    string str1 = "7_";
    string str1a = "7_0";
    string str2 = "mm.root";
    for(int i = 4; i< 37; i++){
        string s = to_string(i);
        string filename;
        if (i<10){
            filename = str1a + s + str2;
        }
        else{
            filename =str1 + s + str2;
        }
        files.push_back(filename);
    }
    
   // read the TTree named "tree" in all root files
    TChain *ch1 = new TChain("tree");
   // Add them into the TChain	
    for (int k = 0 ; k< files.size(); k++){
        ch1->Add(files[k].c_str());
    }
    int entries = ch1->GetEntries();
    cout << entries << endl;
    // Set the address
    ch1->SetBranchAddress("signal",signal);
    
    // loop over all the root files
    //for (int i = 0 ; i< entries ; i++){

      //  ch->GetEntry(i);

    //}




}