If we want Sum of this series up to 2nd term then sum will be:
1-2 =-1
Up to 3rd term:
1-2+3 =2
Up to 4th term
1-2+3-4 = -2
Up to 5th term:
1-2+3-4+5= 3
.
.
.

Now we can conclude that if we want sum up to an Odd term then we always get sum as ((N+1)/2) and if we want sum up to an Even term the sum is in the form of (-1*(N/2)).

Now Let's make logic for this using c programming,

#include <stdio.h>
//function for creating the sum of the
//series up to Nth term
int series_sum(int n)
{
if (n % 2 == 0)
return (-(n / 2));
else
return ((n + 1) / 2);
}
// main code
int main()
{
int n;
printf("Series:1-2+3-4+5-6+7-8.....N\n");
printf("Want some up to N terms?\nEnter the N term:");
scanf("%d", &n);
printf("Sum is:%d", series_sum(n));
return 0;
}

Output

Series:1-2+3-4+5-6+7-8.....N
Want some up to N terms?
Enter the N term:10
Sum is:-5

Let's analyse this problem,

Now we can conclude that if we want sum up to an Odd term then we always get sum as ((N+1)/2) and if we want sum up to an Even term the sum is in the form of (-1*(N/2)).

Now Let's make logic for this using c programming,

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