#pragma GCC optimize("Ofast,inline")
#pragma GCC target("sse,sse2,sse3,ssse3,sse4,sse4.1,sse4.2,popcnt,abm,mmx,avx,avx2,tune=native")
#include<iostream>
#include<algorithm>
#include<cmath>
#include<cstring>
#include<cstdio>
#include<set>
#include<map>
#include<vector>
#include<queue>
#include<deque>
#define ll long long
#define mp make_pair
#define rep(i,x,y) for(int i=(x);i<=(y);++i)
#define r0p(i,n) for(int i=0;i<n;++i)
#pragma GCC optimize(1)
#pragma G++ optimize(1)
#pragma GCC optimize(2)
#pragma G++ optimize(2)
#pragma GCC optimize(3)
#pragma G++ optimize(3)
#pragma GCC optimize("-funsafe-loop-optimizations")
#pragma GCC optimize("-funroll-loops")
#pragma GCC optimize("-fwhole-program")
#pragma GCC optimize("-fthread-jumps")
#pragma GCC optimize("-falign-functions")
#pragma GCC optimize("-falign-jumps")
#pragma GCC optimize("-falign-loops")
#pragma GCC optimize("-falign-labels")
#pragma GCC optimize("-fcaller-saves")
#pragma GCC optimize("-fcrossjumping")
#pragma GCC optimize("-fcse-follow-jumps")
#pragma GCC optimize("-fcse-skip-blocks")
#pragma GCC optimize("-fdelete-null-pointer-checks")
#pragma GCC optimize("-fexpensive-optimizations")
#pragma GCC optimize("-fgcse")
#pragma GCC optimize("-fgcse-lm")
#pragma GCC optimize("-finline-small-functions")
#pragma GCC optimize("-findirect-inlining")
#pragma GCC optimize("-foptimize-sibling-calls")
#pragma GCC optimize("-fpeephole2")
#pragma GCC optimize("-freorder-blocks")
#pragma GCC optimize("-freorder-functions")
#pragma GCC optimize("-frerun-cse-after-loop")
#pragma GCC optimize("-fsched-interblock")
#pragma GCC optimize("-fsched-spec")
#pragma GCC optimize("-fschedule-insns")
#pragma GCC optimize("-fschedule-insns2")
#pragma GCC optimize("-fstrict-aliasing")
#pragma GCC optimize("-fstrict-overflow")
#pragma GCC optimize("-ftree-switch-conversion")
#pragma GCC optimize("-ftree-pre")
#pragma GCC optimize("-ftree-vrp")
#define pii pair<int,int>
#pragma GCC diagnostic error "-std=c++11"
using namespace std;
inline void rdl(long long &val){
    long long x=0;
    int f=1;
    char ch=getchar();
    while((ch<'0' or ch>'9') and ch!='-') ch=getchar();
    if(ch=='-'){
        f=-1;
        ch=getchar();
	}
	while(ch>='0' and ch<='9'){
	    x=(x<<3)+(x<<1)+ch-'0';
		ch=getchar();
	}
    val=x*f;
}
inline void rdi(int &val){
    int x=0;
    int f=1;
    char ch=getchar();
    while((ch<'0' or ch>'9') and ch!='-') ch=getchar();
    if(ch=='-'){
        f=-1;
        ch=getchar();
    }
    while(ch>='0' and ch<='9'){
        x=(x<<3)+(x<<1)+ch-'0';
        ch=getchar();
    }
    val=x*f;
}
inline long long rdl(){
    long long x=0;
    int f=1;
    char ch=getchar();
    while((ch<'0' or ch>'9') and ch!='-') ch=getchar();
    if(ch=='-'){
        f=-1;
        ch=getchar();
    }
    while(ch>='0' and ch<='9'){
        x=(x<<3)+(x<<1)+ch-'0';
        ch=getchar();
    }
    return x*f;
}
inline int rdi(){
    int x=0;
    int f=1;
	char ch=getchar();
	while((ch<'0' or ch>'9') and ch!='-') ch=getchar();
	if(ch=='-'){
        f=-1;
        ch=getchar();
    }
    while(ch>='0' and ch<='9'){
        x=(x<<3)+(x<<1)+ch-'0';
        ch=getchar();
    }
    return x*f;
}
inline void write(int x){
    if(x<0) putchar('-'),x=-x;
    if(x>9) write(x/10);
    putchar(x%10+'0');
}
inline void _write(int x){
	write(x);
	putchar(' ');
}
inline void print(int x){
	write(x);
	putchar('\n');
}
inline void write(ll x){
    if(x<0) putchar('-'),x=-x;
    if(x>9) write(x/10);
    putchar(x%10+'0');
}
inline void _write(ll x){
	write(x);
	putchar(' ');
}
inline void print(ll x){
	write(x);
	putchar('\n');
}
struct dsu{
	int par[200200];
	inline void init(int _n){for(int i=0;i<_n;++i) par[i]=i;}
	inline int find(int now){
		if(par[now]==now) return now;
		return par[now]=find(par[now]);
	}
	inline bool uni(int a, int b){
		int Pa(find(a)),Pb(find(b));
		if(Pa==Pb) return false;
		par[Pa]=Pb;
		return true;
	}
};
int main(){
	int a,b,c;
	cin>>a>>b;
	c=a+b;
	if(c&1) cout<<"IMPOSSIBLE";
	else cout<<c/2;
}

Measured memory_bytes: 256