Algorithm Analysis

If we calculate the sum using a loop, in the worst case (when $a=-2\times 10^9$, $b=2\times 10^9$) we would have to perform up to $4\times 10^9$ iterations, which would lead to Time Limit Exceeded. Let's use the formula for the sum of an arithmetic progression. The first term equals $a$, the last term equals $b$, and the total number of terms is $b-a+1$. Then, the desired sum equals

It should also be noted that the resulting sum may not fit in an int type. Therefore, we should use the long long type for calculation.

Algorithm Implementation

Read the input data. Calculate and output the answer.

scanf("%lld %lld", &a, &b);
res = (a + b) * (b - a + 1) / 2;
printf("%lld\n", res);

Java Implementation

import java.util.*;

public class Main
{
    public static void main(String[] args)
    {
        Scanner con = new Scanner(System.in);
        long a = con.nextLong();
        long b = con.nextLong();
        long res = (a + b) * (b - a + 1) / 2;
        System.out.println(res);
    }
}

Python Implementation

a, b = map(int, input().split())
res = (a + b) * (b - a + 1) // 2
print(res)