我编写了这个简单的程序来对用户编写的扫描int求和，但是当我编译它时，它说“字符串不能转换为int”。该程序有什么问题？

import java.util.*;

public class Pr6{

      public static void main(String[] args){

      Scanner scan = new Scanner (System.in);

      int num1;

      int num2;

      int num3;

      int sum;



                  System.out.print("Please write an integer: ");

       num1 = scan.nextLine();



                  System.out.print("Please write an integer: ");

       num2 = scan.nextLine();



                  System.out.print("Please write an integer: ");

       num3 = scan.nextLine();



       sum = num1 + num2 + num3;

                  System.out.print("Total = " + sum);







        }//main

}//Pr6