function Main(input) {
   var tmp = input.split("\n");
   var origin = tmp[0].split("");
   var next = tmp[1].split("");
   var result = 0;
   
   if(tmp[0]==tmp[1]){
       console.log(result);
   }else{
       for(var i=0; i<tmp[0].length; i++){
           if(origin[i]!==next[i]){
               result+=1;
           }
       }
       console.log(result);
   }
}
Main(require("fs").readFileSync("/dev/stdin", "utf8"));