My code is failing the time limit even though it seems to produce the correct output. How do I decrease the time taken for my code to work?

Question - For a positive integer n let's define a function f: f(n) =  - 1 + 2 - 3 + .. + ( - 1)^n*n Your task is to calculate f(n) for a given integer n.

Input - The single line contains the positive integer n (1 ≤ n ≤ 10^15).

Output - Print f(n) in a single line.

Link to question - https://codeforces.com/problemset/problem/486/A

My code -

int main(){
    ios::sync_with_stdio(0);
    cin.tie(0);

    long long t,sum=0;
    cin >> t;
    for(long long i=1;i<=t;i++){
        if(i%2!=0){
            sum = sum - i;
        }else{
            sum = sum + i;
        }
    }
    cout << sum;
}

The code seems to produce the correct output, however the time limit is a problem. Time limit per test is 1 second. When the input is 1000000000000000 , it says time limit exceeded and fails the test case. How do I reduce the time taken for my code?