C++ :

#include<bits/stdc++.h>
using namespace std;
int a,b;
int main(){
	scanf("%d%d",&a,&b);
	int sum=(a+b)*(b-a+1)/2;
	printf("%d",sum);
	return 0;
}

Python :

# coding=utf-8
n=int(input())
m=int(input())
x=0
for i in range(n,m+1):
    x=x+i
print(x)