const Main = input => {
  const lines = input.split('\n');
  const firstLine = lines[0];
  const secondLine = lines[1];
  const firstLineArray = firstLine.split('');
  const secondLineArray = secondLine.split('');
  const result = firstLineArray.reduce((acc,_,index) => {
    if(firstLineArray[index] == secondLineArray[index]) return acc;
    else return acc + 1;
  },0);
  console.log(result)
}

Main(require("fs").readFileSync("/dev/stdin", "utf8"));