if([matrix count] == 2 && [[matrix objectAtIndex:0]count] == 2){
        double a1 = [[[matrix objectAtIndex:0]objectAtIndex:0]doubleValue];
        double a2 = [[[matrix objectAtIndex:0]objectAtIndex:1]doubleValue];
        double b1 = [[[matrix objectAtIndex:1]objectAtIndex:0]doubleValue];
        double b2 = [[[matrix objectAtIndex:1]objectAtIndex:1]doubleValue];
        return a1*b2 - a2*b1;
    }