#include<stdio.h>

int main()

{

int i=1,sum=0,flag=1;

for(i=1;i<=100;i++)

{

if(i%2!=0)

{

sum+=i;

}

else if(i%2==0)

{

flag=-flag;

sum=sum+i*flag;

}

}

printf("%d\n",sum);

return 0;

}