#include<stdio.h>

int calculateSum(int start,int end){
    int n =end-start+1;
    return n *(start+end)/2;
}
int main(){
    int start, end;
    if(scanf("%d", &start)!=1 || scanf("%d", &end)!=1){
        printf("Invalid input\n");
        return 1;
    }
    if(start>end){
        printf("Invalid input\n")
        return 0;
     }
     printf("%d\n", calculateSum(start,end));
     return 0;
}