"use strict";
const Main = (input) =>{
   let xs = input.trim().split('');
   let xs1 = input.trim().split(' ');
   //console.log(xs.length);
   let result = "";
   for (let i = 0 ; i < xs.length; i++) {
   let pos = xs.length - i - 1;
   result += xs1[0].charAt(pos);
   }
   let count = 0;
   for(let j =0; j< xs.length; j++){
       if(result[j]==xs[j]){
          count++
       }else{
          count += 0
       }
   }
   console.log((xs.length-count)/2);
}
Main(require('fs').readFileSync('/dev/stdin','utf8'));