Idea
Sigma
Code
#define _CRT_SECURE_NO_WARNINGS
#include <stdio.h>
int main(void) {
long long A, B;
scanf("%lld %lld", &A, &B);
if (A < B) {
printf("%lld", (A + B) * (B - A + 1) / 2);
}
else {
printf("%lld", (A + B) * (A - B + 1) / 2);
}
return 0;
}
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