'use strict'
const main = (input) => {
  const el = input.trim().split(" ").map(el => el * 1)
  let ans = - 1
   for(let i = 0 ;i < 100000; i++){
  	if (Math.floor(0.08 * i)  == el[0] && Math.floor(0.1 * i) == el[1]) {
    	ans = i;
      	break;
    }
  }
    console.log(ans)
  
}
 
main(require('fs').readFileSync('/dev/stdin', 'utf-8'))