//Man always remember love because of romance only!
#include<bits/stdc++.h>
using namespace std;
inline int read(){
    int X=0,w=0; char ch=0;
    while(!isdigit(ch)) {w|=ch=='-';ch=getchar();}
    while(isdigit(ch)) X=(X<<3)+(X<<1)+(ch^48),ch=getchar();
    return w?-X:X;
}
inline void write(int x){
    if(x<0) putchar('-'),x=-x;
    if(x>9) write(x/10);
    putchar(x%10+'0');
}
int main(){
    int l=read(),r=read();
    if(l+1==r) puts("-1");
    else cout<<(l+r+1)/2;
    return 0;
}