#include <stdio.h>
int main(void) {
    int start, end;
    int sum = 0;
    if (scanf("%d", &start) != 1 || scanf("%d" , &end) !=
    printf("Invalid input");
    return 0;
} 
if (start > end) {
    printf("Invalid input");
    return 0;
}
for ( int i = start; i<= end; i++) {
    sum += i;
 }
printf("%d" , sum);
return 0;
}