function main(input){
  let s = input.split("\n")[0];
  let t = input.split("\n")[1];
  let sArray = s.split("");
  let tArray = t.split("");
  let count = 0;
  
  for(let i=0; i<sArray.length; i++){
    if(sArray[i]!==tArray[i]){
      count++;
    }
  }
  console.log(count);
}
main(require("fs").readFileSync("/dev/stdin", "utf8"));