#include <stdio.h>

// Sum of Arithmetic Sequence
// an = a1 + (n-1) * d
// Sn = (a1 + an) * n/2

int main(void) {

    int d, a1, n, an, Sn;

    printf("Sum of Arithmetic Sequence!\n");

    printf("a1:");
    scanf("%d", &a1);

    printf("d:");
    scanf("%d", &d);

    printf("n:");
    scanf("%d", &n);


    //an = a1 + (n-1) * d;
    //Sn = (a1 + an) * n/2;

    Sn = (a1 + (a1+(n-1)*d) ) * n/2;

    printf("Sn := %d\n", Sn);


    return 0;
}