Dev c++ Hesap makinesi

#include <iostream> 
  #include <stdlib.h> 
  #include <math.h> 
  using std::cout; 
  using std::cin; 
  using std::endl; 
  int main() 
  { 
  int n; 
  long int a,b; 
  cout<<"Aşagidaki islemlerden birini seciniz:"<<endl<<endl; 
  cout<<"Toplama islemi=1"<<endl; 
  cout<<"Cikarma islemi=2"<<endl; 
  cout<<"Carpma islemi=3"<<endl; 
  cout<<"Bolme islemi=4"<<endl; 
  cout<<"Karekok islemi=5"<<endl; 
  cout<<"Kare alma islemi=6"<<endl; 
  cin>>n; 
  if(n>=5) 
  { 
  cout<<"Sadece tek sayi giriniz"<<endl; 
  cin>>a; 
  } 
  else 
  { 
  cout<<"ilk sayi'yi giriniz"<<endl; 
  cin>>a; 
  cout<<"ikinci sayi'yi giriniz"<<endl; 
  cin>>b; 
  } 
   
   
  switch(n) { 
  case 1:{cout<<"Sonuc="<<a+b<<endl;break;} 
  case 2:{cout<<"Sonuc="<<a-b<<endl;break;} 
  case 3:{cout<<"Sonuc="<<a*b<<endl;break;} 
  case 4:{cout<<"Sonuc="<<a/b<<endl;break;} 
  case 5:{cout<<"Sonuc="<<sqrt(a)<<endl;break;} 
  case 6:{cout<<"Sonuc="<<a*a<<endl;break;} 
  default: 
  cout<<"yanlis tusa bastin"<<endl;break; 
  cout<<"----"<<endl<<endl; 
  system("PAUSE"); 
  return 99; 
  } 
  }