import Foundation
func toTuple<T>(_ args: [T]) -> (T, T) { (args[0], args[1]) }
func readInts() -> [Int] { readLine()!.split(separator: " ").map { Int($0)! } }

let (a, b) = toTuple(readInts())
let n = b - a - 1
print(n * (n + 1) / 2 - a)