int sum=0;
  if(gender=='b'){
      for(int i=0;i<number_of_students;i+=2){
          sum+=marks[i];
      }
    return sum;
  }
  else{
      for(int i=1;i<number_of_students;i+=2){
          sum+=marks[i];    
      }
    return sum;
  }
}