#include<stdio.h>

int main(){

int a,b;
scanf("%d %d",&a,&b);
//1.a是中间项
int c1=a-b+a;
//2.b是中间项
int c2=b-a+b;
//3.c是中间项，先确保c是整数
int c3;
if((a+b)%2==0)
c3=(int)(a+b)/2;
printf("%d",c1);

}