import std.stdio, std.conv, std.string, std.array, std.math, std.regex, std.range, std.ascii, std.numeric;
import std.typecons, std.functional, std.traits;
import std.algorithm, std.container;
import core.stdc.stdlib, core.bitop;

void main()
{
    auto A = scanElem;
    auto B = scanElem;

    if(abs(A+B)%2!=0){
        writeln("IMPOSSIBLE");
        return;
    }

    writeln(abs(A+B)/2);
}

enum INF = long.max/5;
enum MOD = 10^^9+7L;

bool isInf(long v) pure
{
    return v>=INF;
}

long mod(long n, long mod) pure
{
    return (n%mod+mod)%mod;
}

alias MInt = ModInt!MOD;

struct ModInt(alias Mod)
if(isIntegral!(typeof(Mod)) && isPrime(Mod))
{
    long value;

    this(ModInt!Mod v)
    {
        value = v.value;
    }
    this(long v)
    {
        value = mod(v, Mod);
    }

    private static ModInt!Mod inv(ModInt!Mod v) pure
    {
        ModInt res=1;
        long p = Mod-2;
        while(p!=0)
        {
            if(p&1)res *= v;
            v *= v;
            p >>= 1;
        }
        return res;
    }

    ModInt!Mod opBinaryRight(string op, T)(T v) const
    if((op=="+"||op=="-"||op=="*")&&isIntegral!T)
    {
        return ModInt!Mod(mixin("v"~op~"value"));
    }
    ModInt!Mod opBinary(string op, T)(T v) const
    if((op=="+"||op=="-"||op=="*")&&(is(T==ModInt!Mod)||isIntegral!T))
    {
        return ModInt!Mod(mixin("value"~op~"v"));
    }

    ModInt!Mod opBinaryRight(string op, T)(T v) const
    if((op=="/")&&isIntegral!T)
    {
        return v * inv(this);
    }
    ModInt!Mod opBinary(string op, T)(T v) const
    if((op=="/")&&(is(T==ModInt!Mod)||isIntegral!T))
    {
        return this * inv(ModInt(v));
    }

    void opAssign(T)(T v)
    if(isIntegral!T)
    {
        this = ModInt!Mod(v);
    }

    void opOpAssign(string op, T)(T v)
    {
        this = mixin("this"~op~"v");
    }

    bool opEquals(T)(const T v) const
    if(is(T==ModInt!Mod)||isIntegral!T)
    {
        return value == ModInt(v).value;
    }

    long opCast() const {
        return value;
    }

    string toString() const {
        return value.to!string;
    }
}
unittest
{
    assert(is(ModInt!MOD));
    assert(is(ModInt!17));
    assert(!is(ModInt!0));
    assert(!is(ModInt!10));
}
unittest
{
    alias MInt = ModInt!13;
    assert(MInt.inv(MInt(4))==10);
    assert(MInt.inv(MInt(6))==11);
    assert(MInt.inv(MInt(12))==12);
}
unittest
{
    alias MInt = ModInt!13;
    MInt value;
    value = MInt(14) + MInt(18);
    assert(value==6);
    value = 14 - MInt(28);
    assert(value==12);
    value = MInt(17) * -19;
    assert(value==2);

    value = MInt(7) / 4;
    assert(value==5);
    value = 8 / MInt(4);
    assert(value==2);
    value = 9;
    value /= 4;
    assert(value==12);

    value = 29-MInt(16);
    assert(value==0);
    value = MInt(13)*5;
    assert(value==0);
    value = 0;
    assert(value==0);

    value = 3;
    value += MInt(11);
    assert(value==1);
    value -= 7;
    assert(value==7);
    value *= MInt(4);
    assert(value==2);
    value = 23;
    assert(value == cast(long)value);
}
unittest
{
    ModInt!MOD value;
    value = MOD-1;
    assert(value==MOD-1);
    value = MOD;
    assert(value==0);
}

struct Grid(T){
    private T[] grid;
    size_t width, height;
    private size_t stride;

    private this(T[] g, size_t w, size_t h, size_t s)
    {
        grid = g;
        width = w;
        height = h;
        stride = s;
    }

    this(size_t w, size_t h)
    {
        this(new T[w*h], w, h, w);
    }

    void setRow(InputRange)(size_t y, InputRange range)
    if(isInputRange!InputRange && is(typeof(T.init=range.front)))
    {
        foreach(x; 0..width)
        {
            assert(!range.empty);
            this[x, y] = range.front;
            range.popFront;
        }
    }

    void setCol(InputRange)(size_t x, InputRange range)
    if(isInputRange!InputRange && is(typeof(T.init=range.front)))
    {
        foreach(y; 0..height)
        {
            assert(!range.empty);
            this[x, y] = range.front;
            range.popFront;
        }
    }

    Grid!T copy(){
        if(width == stride){
            return Grid!T(grid.dup, width, height, stride);
        }

        auto res = Grid!T(width, height);
        foreach(y;0..height)
        foreach(x;0..width)
        {
            res[x, y] = this[x, y];
        }
        return res;
    }

    ref T opIndex(size_t x, size_t y)
    {
        return grid[x+stride*y];
    }
    ref T opIndex(E)(E e)
        if(
            isIntegral!(typeof(e.x)) &&
            isIntegral!(typeof(e.y)))
    {
        return this[e.x, e.y];
    }

    Grid!T opIndex(size_t[2] r1, size_t[2] r2)
    {
        auto startOffset = r1[0] + r2[0]*stride;
        auto endOffset = r1[1] + (r2[1] - 1)*stride;

        auto resGrid = grid[startOffset .. endOffset];
        auto resStride = stride;
        auto resWidth = r1[1] - r1[0];
        auto resHeight = r2[1] - r2[0];
        return Grid!T(resGrid, resWidth, resHeight, resStride);
    }
    
    Grid!T opIndex(size_t[2] r1, size_t j) { return opIndex(r1, [j, j+1]); }
    Grid!T opIndex(size_t i, size_t[2] r2) { return opIndex([i, i+1], r2); }

    size_t[2] opSlice(size_t dim)(size_t start, size_t end)
    if (dim == 0 || dim == 1)
    // in { assert(start >= 0 && end <= this.opDollar!dim); }
    body{
        return [start, end];
    }

    size_t opDollar(size_t dim : 0)() const { return width; }
    size_t opDollar(size_t dim : 1)() const { return height; }

    string toString() {
        long count;
        foreach(y; 0..height)
        {
            foreach(x; 0..width)
            {
                long eCount = this[x, y].to!string.length;
                static if(is(typeof(this[x,y].isInf)))
                {
                    if(this[x,y].isInf)
                    {
                        eCount = 1;
                    }
                }
                count = max(count, eCount);
            }
        }
        count++;

        string res = "\n";
        foreach(y; 0..height)
        {
            foreach(x; 0..width)
            {
                string joinStr = this[x, y].to!string;
                static if(is(typeof(this[x,y].isInf)))
                {
                    if(this[x,y].isInf)
                    {
                        joinStr = "*";
                    }
                }
                foreach(i;0..count-joinStr.length)
                {
                    res ~= ' ';
                }
                res ~= joinStr;
            }
            res ~= '\n';
        }
        return res;
    }
}

long lcm(long a, long b) pure
{
    return a * b / gcd(a, b);
}

class UnionFind{
    UnionFind parent = null;

    void merge(UnionFind a)
    {
        if(same(a)) return;
        a.root.parent = this.root;
    }
    UnionFind root()
    {
        if(parent is null)return this;
        return parent = parent.root;
    }
    bool same(UnionFind a)
    {
        return this.root == a.root;
    }
}

enum TermColor{
    Blue,
    Red,
    Purple,
    Gray,
    Green,
    Brown,
    Default,
}

string termStr(TermColor c) pure
{
    switch(c)
    {
        case TermColor.Blue:   return "\033[38;2;122;158;194m";
        case TermColor.Red:    return "\033[38;2;204;130;66m";
        case TermColor.Purple: return "\033[38;2;158;123;176m";
        case TermColor.Gray:   return "\033[38;2;106;135;89m";
        case TermColor.Green:  return "\033[38;2;165;194;97m";
        case TermColor.Brown:  return "\033[38;2;204;130;66m";
        default: return "\033[0m";
    }
}

void print(TermColor C = TermColor.Gray, T...)(T t)
{
    debug stderr.write(C.termStr, t, TermColor.Default.termStr);
}
void printf(TermColor C = TermColor.Gray, T...)(T t)
{
    debug stderr.write(C.termStr);
    debug stderr.writef(t);
    debug stderr.write(TermColor.Default.termStr);
}
void println(TermColor C = TermColor.Gray, T...)(T t)
{
    debug stderr.writeln(C.termStr, t, TermColor.Default.termStr);
}
void printfln(TermColor C = TermColor.Gray, T...)(T t)
{
    debug stderr.write(C.termStr);
    debug stderr.writefln(t);
    debug stderr.write(TermColor.Default.termStr);
}

// debug{
//     void write(T...)(T t)
//     {
//         stdout.write(TermColor.Blue.termStr, t, TermColor.Default.termStr);
//     }
//     void writeln(T...)(T t)
//     {
//         stdout.writeln(TermColor.Blue.termStr, t, TermColor.Default.termStr);
//     }
//     void writefln(T...)(T t)
//     {
//         stdout.writefln(TermColor.Blue.termStr, t, TermColor.Default.termStr);
//     }
// }

T[] scanArray(T = long)()
{
    return readln.split.to!(T[]);
}

char scanChar()
{
    int c = ' ';
    while (isWhite(c) && c != -1)
    {
        c = getchar;
    }
    return cast(char)c;
}
T scanElem(T = long)()
{
    char[] res;
    int c = ' ';
    while (isWhite(c) && c != -1)
    {
        c = getchar;
    }
    while (!isWhite(c) && c != -1)
    {
        res ~= cast(char) c;
        c = getchar;
    }
    return res.strip.to!T;
}

alias scanString = scanElem!string;

template fold(fun...) if (fun.length >= 1)
{
    auto fold(R, S...)(R r, S seed)
    {
        static if (S.length < 2)
        {
            return reduce!fun(seed, r);
        }
        else
        {
            import std.typecons : tuple;

            return reduce!fun(tuple(seed), r);
        }
    }
}

struct Factor
{
    long n;
    long c;
}

//素因数分解
Factor[] factors(long n) pure
{
    Factor[] res;
    for (long i = 2; i ^^ 2 <= n; i++)
    {
        if (n % i != 0)
            continue;

        long c;
        while (n % i == 0)
        {
            n = n / i;
            c++;
        }
        res ~= Factor(i, c);
    }
    if (n != 1)
        res ~= Factor(n, 1);

    return res;
}
//約数をすべて列挙
long[] divisors(long n) pure
{
    long[] list;
    void func(Factor[] fs, long n)
    {
        if(fs.empty){
            list ~= n;
            return;
        }

        foreach(c; 0..fs[0].c+1)
        {
            func(fs[1..$], n * (fs[0].n ^^ c));
        }
    }

    func(factors(n), 1);
    sort(list);
    return list;
}
//nまでの素数のリスト
long[] primes(long n) pure
{
    if(n<2)return [];

    auto table = new long[cast(size_t)n+1];

    long[] res;
    for(size_t i = 2;i<=n;i++)
    {
        if(table[i]==-1) continue;
        for(size_t a = i;a<table.length;a+=i)
        {
            table[a] = -1;
        }
        res ~= i;
    }
    return res;
}
//素数判定
bool isPrime(long n) pure
{
    if (n <= 1)
        return false;
    if (n == 2)
        return true;
    if (n % 2 == 0)
        return false;
    for (long i = 3; i ^^ 2 <= n; i += 2)
        if (n % i == 0)
            return false;
    return true;
}

Measured memory_bytes: 256