how should I can fix this error: no match for 'operator<' and '

    #include<iostream>
    using namespace std;
   
 class enemy{                       //this is my base class
        protected:             
        int attack;
    public:
        void attackpower(int a){
            attack=a;
        }
    
    };
   
 class ninja :public enemy{          // this is my first derived class
    public:
        void attack()
        {cout<< " i am a ninja with " <<  attack << "power" << endl;}
    
    };
    
    class monster: public enemy{       ////this is my second derived class
      void attack()
      {
          cout<<"i am a monster with "<< attack << "power"<<endl;
          }
    
    };
    //main programs start
    int main(){
    ninja n;
    monster m;
    enemy *enemy1=&n;
    enemy *enemy2=&m;
    enemy1->attackpower(10);
    enemy2->attackpower(90);
    n.attack();
    m.attack();
    }

This is the error line

    cout<<"i am a monster with "<< attack << "power"<<endl;