#include <stdio.h>
int main(void)
{
    long long int a, l, count=0, i;
    scanf("%lld %lld", &a, &l);
    for(i=a;i<=l;i++){
        count++;
    }
    long long int sum = (count*0.5)*(a+l);
    printf("%lld\n", sum);
    return 0;
}