Error Incompatible types: possible lossy conversion from double to int

Question

When I run the below code, I get error at res1 = Math.ceil((double) dividend / divisor);

Incompatible types: possible lossy conversion from double to int

However, I do not get the error when I do the same operation but this time add the result to an integer res1.

res1 += Math.ceil((double) dividend / divisor);

Can someone pls help understand this more?

void func(int[] dividends , int divisor){
  int res1 = 0;

  for(int dividend : dividends){  
     res1 = Math.ceil((double) dividend / divisor);   //error: incompatible types: possible lossy conversion from double to int

     res1 += Math.ceil((double) dividend / divisor);  //this works

     print(res1);
  }
}