Slightly-off output in arithmetic sum

[GonlyG]

Everything explained in code comments.

#include <iostream>
using namespace std;
int main() {
/* first number is amount of triplet input [A B N];
A is the first value of the sequence, 
B is the step size and 
N is the number of first values which should be accounted. 
calculate the sum of first members of arithmetic sequence.
Sn=n/2(a1 + an); an = a1 + (n - 1)d, */

int a, n, sum, firstnum, lastnum, d;
cin >> a;
for (int i =0; i<a; i++){
    cin>> firstnum;
    cin>>d;
    cin>> n;
    lastnum = firstnum + (n-1)*d;
    sum = (n/2)*(firstnum + lastnum);
    cout << sum<<" ";}
}

Input: 8
18 5 93
13 5 29
14 11 46
12 5 35
18 17 74
25 11 28
26 18 75
1 7 80
Output: 22816 2324 12029 3298 47249 4858 51208 22200
Expected: 23064 2407 12029 3395 47249 4858 51900 22200
note that, first, second, fourth, and seventh value were off, but the others weren't. Anyone care to explain?

[JayhawkZombie]

sum = (n/2)*(firstnum + lastnum);

Integer division truncates towards zero. 5 / 7 = 0 in integer division. You probably don't want this. I fixed this and the output was the expected output.

[GonlyG]

double fixed it, thanks.