import java.util.Scanner;
class Main{
  public static void main(String[] args) {
    Scanner sc = new Scanner(System.in);
    int a = sc.nextInt();
    int b = sc.nextInt();
    sc.close();
    int[] arr = new int[1];
    
    for(int i = 1; i < 10000000; i++) {
      if(i*8/100 == a && b == i*10/100) {
        arr[0] = i;
        break;
      }
    }
    
    if(arr[0] == 0) {
      System.out.println("-1"); 
    } else {
      System.out.println(arr[0]); 
    }
  }
}
/tmp/hf-datasets-cache/medium/datasets/42704881117311-config-parquet-and-info-qiankunmu-Project_CodeNet-896c9d36/hub/datasets--qiankunmu--Project_CodeNet_Python800_and_Java250/snapshots/c899fde44a77ba1cc14973ed3de064e4ff3aadc7/Project_CodeNet_Java250.tar.gz