What is wrong with that?
package pl.codegym.task.task05.task0507;
/*
Średnia arytmetyczna
*/
import java.io.PrintStream;
import java.util.Scanner;
import static java.lang.System.*;
public class Solution {
public static void main(String[] args) throws Exception {
Scanner scanner = new Scanner(in);
float suma = 0;
float ilosc = 0;
while(true){
int i = scanner.nextInt();
if(i == (-1))
break;
ilosc++;
suma += i;
}
float wynik = suma/ilosc;
System.out.printf("%.1f%n", wynik);
}
}