#include<stdio.h>
int main()
{
    int start_price, end_price;
    long long  total_sum;
    if(scanf("%d",&start_price) !=1)
    {
        return 1;
    }
    if(scanf("%d",&end_price) !=1)
    {
        return 1;
    }
    if(start_price > end_price)
    {
        printf("Invalid input\n");
        return 0;
    }
    long long n=(long long)end_price - start_price+1;
    total_sum = n*((long long)start_price+end_price)\2;
    
    printf("%lld\n", total_sum);
    return 0;
}