#include <stdio.h>

int main() {
    int start, end;
    scanf("%d", &start);
    scanf("%d", &end);
    
    if (start > end) {
        printf("Invaild input\n");
        return 0;
    }
    
    //Sum of intergers from start to end using formula
    int n = end - start + 1;          // number of terms
    int sum = n * (start + end) / 2;  // arithmetic sum formula
    
    print("%d\n", sum);
    
    return 0;
}