Write a program in C++ to find out the sum of an A.P. series

#include <iostream>
using namespace std;

int main()
{
    int n1, df, n2, i, ln;
    int s1 = 0;
    cout << "\n\n Find out the sum of A.P. series:\n";
    cout << "-----------------------------------------\n";
    cout << " Input  the starting number of the A.P. series: ";
    cin >> n1;
    cout << " Input the number of items for  the A.P. series: ";
    cin >> n2;
    cout << " Input  the common difference of A.P. series: ";
    cin >> df;
    s1 = (n2 * (2 * n1 + (n2 - 1) * df)) / 2;
    ln = n1 + (n2 - 1) * df;
    cout << " The Sum of the  A.P. series are : " << endl;
    for (i = n1; i <= ln; i = i + df) 
    {
        if (i != ln)
            cout << i << " + ";
        else
            cout << i << " = " << s1 << endl;
    }
}