Calculating a number from items in an array then getting the sum of those numbers with Array.prototype.reduce()? (Javascipt)

I've been asked in an exercise to find out how many years did all the inventors live in one number using "Array.prototype.reduce()"

const inventors = [
  { first: 'Albert', last: 'Einstein', year: 1879, passed: 1955 },
  { first: 'Isaac', last: 'Newton', year: 1643, passed: 1727 },
  { first: 'Galileo', last: 'Galilei', year: 1564, passed: 1642 },
  { first: 'Marie', last: 'Curie', year: 1867, passed: 1934 },
  { first: 'Johannes', last: 'Kepler', year: 1571, passed: 1630 },
  { first: 'Nicolaus', last: 'Copernicus', year: 1473, passed: 1543 },
  { first: 'Max', last: 'Planck', year: 1858, passed: 1947 },
  { first: 'Katherine', last: 'Blodgett', year: 1898, passed: 1979 },
  { first: 'Ada', last: 'Lovelace', year: 1815, passed: 1852 },
  { first: 'Sarah E.', last: 'Goode', year: 1855, passed: 1905 },
  { first: 'Lise', last: 'Meitner', year: 1878, passed: 1968 },
  { first: 'Hanna', last: 'Hammarström', year: 1829, passed: 1909 }
]; 

So far I've come to this which lists each age individually but now I'm unsure how to actually add the numbers up to return as 1 integer.

console.log("Exercise 5:")
const totalYears = inventors.reduce((acc, inventor) => {
  let age = inventor.passed - inventor.year

  acc[`${inventor.first} ${inventor.last}`] = age
  return acc
}, {})
console.log(totalYears)

Any help would be fantastic, thank you