int minimumCost(coupon_t coupons[], int numCoupons, int units){

    int i;
    int cost= -1; temp;
    if (!units) return 0; //base case when you are able to buy all units by using all coupons

    for(i=numCoupons-1; i>-1; i++){ //we count the minimum for each of the'i'-th coupon that is the largest indexed coupon to be
                    //used
        if( units>=coupons[i].quantity &&
        (temp = minimumCost(coupons, i, units - coupons[i].quantity)>=0 )
        //recursion using i because we now consider the first i-1 tickets; we assumed ith coupon is largest indexed coupon      //would be used. Also if temp = -1, we skip since no viable solution
            temp += coupons[i].price; //need to add on coupons[i].price since we are using it;

        if(cost = -1 || (temp < cost && temp!=-1))
            cost = temp;
    }

    return cost;
}