// @ts-check

read(function(err, input) {
  var args = input.split(' ');
  var a = Number(args[0]);
  var b = Number(args[1]);
  harmony(a, b);
});

/**
 * @param {(err: Error | null, input?: string) => void} callback
 */
function read (callback) {
  var input = '';
  process.stdin.setEncoding('utf8');
  process.stdin.on('readable', () => {
    /** @type {string | Buffer} */
    var chunk;
    while ((chunk = process.stdin.read()) !== null) {
      input += chunk;
    }
  });

  process.stdin.on('end', () => {
    callback(null, input);
  });
  process.stdin.on('error', () => {
    var error = new Error('Failed to read from stdin');
    callback(error);
  });
}

/**
 * @param {number} a
 * @param {number} b
 */
function harmony(a, b) {
  var s = (a + b);
  if (s % 2 === 0) {
    var k = s / 2;
    console.log(k);
  } else {
    console.log('IMPOSSIBLE');
  }
}
