#include<stdio.h>
int main() {
long long int a, b, c, e;
int d = 2;
scanf("%lld %lld", &a, &b);
if (a % 2 == 0 && b % 2 == 0 ){
c = (b - a)/d + 1;
e = (a + b) / 2 * c;
printf("%lld",e);
}
else if (a % 2 != 0 && b % 2 == 0 ){
a = a + 1 ;
c = (b - a)/d + 1;
e = (a + b) / 2 * c;
printf("%lld",e);
}
else if (a % 2 != 0 && b % 2 != 0 ){
a = a + 1 ;
b = b - 1 ;
c = (b - a)/d + 1;
e = (a + b) / 2 * c;
printf("%lld",e);
}
return 0;
}
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