input = require('fs').readFileSync('/dev/stdin', 'utf8')
lines = input.split('\n')
line = lines.shift()
A = line.split(' ')[0] * 1
B = line.split(' ')[1] * 1
ans = (A + B) / 2 == ~~((A + B) / 2) ? (A + B) / 2 : 'IMPOSSIBLE'  
console.log(ans)