process.stdin.resume()
process.stdin.setEncoding('utf8')

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

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

let numArray = input_string.split(' ')
let numA = Math.abs(numArray[0])
let numB = Math.abs(numArray[1])
let addNum = numA + numB

if(addNum%2 == 0){
  reader.on('close', () => {
      console.log(addNum/2)
  })
}else{
  reader.on('close', () => {
      console.log('IMPOSSIBLE')
  })
}

