问React组件加法使我获得NaN

EN

const Total = (props) => {
  return (
    <div>
      <p>Number of exercises {props.exercises1 + props.exercises2 + props.exercises3}</p>
    </div>
  )
}

const App = () => {
  const course = 'Half Stack application development'
  const part1 = {
    name: 'Fundamentals of React',
    exercises: 10
  }
  const part2 = {
    name: 'Using props to pass data',
    exercises: 7
  }
  const part3 = {
    name: 'State of a component',
    exercises: 14
  }


  return (
    <div>
      <Total exercise1={part1.exercises} exercises2={part2.exercises} exercises3={part3.exercises} />
    </div>
  )
}

export default App