#include <bits/stdc++.h>
#include <iostream>
#include <algorithm>
#include <vector>
#define mod 1000000007
#define DEBUG(x) cout << '>' << #x << ':' << x << endl;
#define REP(i, n) for (int i = 0; i < (n); i++)
#define FOR(i, a, b) for (int i = (a); i <= (b); i++)
#define FORD(i, a, b) for (int i = (a); i >= (b); i--)
#define pb push_back
#define eb emplace_back
#define mp make_pair
inline bool EQ(double a, double b) { return fabs(a - b) < 1e-9; }
const int INF = 1 << 29;
typedef long long ll;
inline int two(int n) { return 1 << n; }
inline int test(int n, int b) { return (n >> b) & 1; }
inline void set_bit(int &n, int b) { n |= two(b); }
inline void unset_bit(int &n, int b) { n &= ~two(b); }
inline int last_bit(int n) { return n & (-n); }
inline int ones(int n)
{
   int res = 0;
   while (n && ++res)
      n -= n & (-n);
   return res;
}
template <class T>
void chmax(T &a, const T &b) { a = max(a, b); }
template <class T>
void chmin(T &a, const T &b) { a = min(a, b); }

long long int gcd(long long int a, long long int b)
{
   if (b == 0)
      return a;
   return gcd(b, a % b);
}

long long int lcm(long long int a, long long int b)
{
   return (a * b) / gcd(a, b);
}

long long int max(long long int a, long long int b)
{
   if (a > b)
      return a;
   return b;
}

long long int power(long long int x, long long int y)
{
   long long int res = 1;
   while (y > 0)
   {
      if (y & 1)
         res = res * x;
      y = y >> 1;
      x = x * x;
   }
   return res;
}

long long int powermod(long long int x, long long int y)
{
   long long int res = 1;
   x = x % mod;
   while (y > 0)
   {
      if (y & 1)
         res = (res * x) % mod;
      y = y >> 1;
      x = (x * x) % mod;
   }
   return res;
}

int maxSubArraySum(int a[], int size)
{
   int max_so_far = INT_MIN, max_ending_here = 0;

   for (int i = 0; i < size; i++)
   {
      max_ending_here = max_ending_here + a[i];
      if (max_so_far < max_ending_here)
         max_so_far = max_ending_here;

      if (max_ending_here < 0)
         max_ending_here = 0;
   }
   return max_so_far;
}

long long mulmod(long long a, long long b)
{
   long long res = 0;
   a %= mod;
   while (b)
   {
      if (b & 1)
         res = (res + a) % mod;
      a = (2 * a) % mod;
      b >>= 1;
   }

   return res;
}

bool isPrime(long long int n)
{
   if (n <= 1)
      return false;
   if (n <= 3)
      return true;
   if (n % 2 == 0 || n % 3 == 0)
      return false;
   for (int i = 5; i * i <= n; i += 6)
   {
      if ((n % i == 0) || (n % (i + 2) == 0))
         return false;
   }
   return true;
}

long double dist(long long int x1, long long int y1, long long int x2, long long int y2)
{
   return (long double)sqrt((long double)((x1 - x2) * (x1 - x2) + (y1 - y2) * (y1 - y2)));
}
long long int squaredist(long long int x1, long long int y1, long long int x2, long long int y2)
{
   return ((x1 - x2) * (x1 - x2) + (y1 - y2) * (y1 - y2));
}

long long int nCr(long long int n, long long int r)
{
   if (r == 0)
      return 1;
   return (n * nCr(n - 1, r - 1)) / r;
}

long long int countDivisors(long long int n)
{
   long long int cnt = 0;
   for (int i = 1; i <= sqrt(n); i++)
   {
      if (n % i == 0)
      {
         if (n / i == i)
            cnt++;
         else
            cnt = cnt + 2;
      }
   }
   return cnt;
}

int xorRange(int x, int y)
{
   if (x % 2 == 0)
   {
      int pattern[] = {y, 1, y + 1, 0};
      return pattern[(int)((y - x) % 4)];
   }
   else
   {
      int pattern[] = {x, x ^ y, x - 1, (x - 1) ^ y};
      return pattern[(int)((y - x) % 4)];
   }
}

using namespace std;

int main()
{
   ios_base::sync_with_stdio(0);
   cin.tie(0);
   cout.tie(0);
   ll a, b;
   cin >> a >> b;
   ll x = (b - a - 1) * (b - a) / 2;
   cout << x - a << "\n";
   return 0;
}

Measured memory_bytes: 256