It is highly advisable to go through Command Line Arguments Post before even looking at the code. Please study this for TCS and come back to this post later.

#include <stdio.h>

int main(int argc, char *argv[])

{

             int base, height;

            float area;

             base = atol(argv[1]);

            height = atol(argv[2]);

            area = base*height/2;

            printf("%f",area);

}