const main = args =>{
  const s0 = args[0];
  const s1 = args[1];
  let result = 0;
  for(let i=0;i<s0.length;i++){
  	if(s0[i]!==s1[i]){
    	result++;
    }
  }
  console.log(result);
}

 
main( require("fs").readFileSync("/dev/stdin", "utf8").trim().split("\n") );