"use strict"
process.stdin.resume()
process.stdin.setEncoding('utf8')

let numArray = "";
let reader = require('readline').createInterface({
    input: process.stdin,
    output: process.stdout
})

reader.on('line', (line) => {
    numArray = line.split(' ');
})

reader.on('close', () => {
    let numA = Math.abs(numArray[0])
    let numB = Math.abs(numArray[1])
    let addNum = numA + numB
    
    if(addNum%2 == 0){
          console.log(addNum/2)
    }else{
          console.log('IMPOSSIBLE')
    }
})
