/**
 * rpn.c - a simple Reverse Polish Notation calculator that reads its
 *   input from standard input.
 */

#include <stdio.h>
#include <string.h>
#include <stdlib.h>
#include <limits.h>

// +-----------+------------------------------------------------------
// | Constants |
// +-----------+

// The capacity of the stack
#define STACK_CAPACITY 16


// +------+-----------------------------------------------------------
// | Main |
// +------+

int 
main (int argc, char *argv[])
{
  int stack[STACK_CAPACITY] = { 0 }; 
                        // The stack of numbers for RPN evaluation
  int top = 0;          // The top of the stack.  That is, the first
                        //   empty space on the stack.  Also the number
                        //   of items on the stack.
  int done = 0;         // A flag that indicates whether or not we're
                        //   done processing input.
  int i;                // The next input value.
  int ch;               // A character we're reading
  int scanok;          // The result of scanf
  while (!done)
    {
      i = 0;

      if ((scanok = (scanf (" p%n",&i) != EOF)) && (i >= 1))
        {
          if (top == 0)
            printf ("NO VALUE\n");
          else
            printf ("%d\n", stack[top-1]);
        }
      else if (scanok && (scanok = (scanf (" +%n",&i) != EOF)) && (i >= 1))
        {
          top--;
          if (top <= 0)
            {
              printf ("ERROR: Stack underflow\n");
              return EXIT_FAILURE;
            }
          stack[top-1] = stack[top-1] + stack[top];
        } // "+"
      else if (scanok && (scanok = (scanf (" -%n",&i) != EOF)) && (i >= 1))
        {
          top--;
          if (top <= 0)
            {
              printf ("ERROR: Stack underflow\n");
              return EXIT_FAILURE;
            }
          stack[top-1] = stack[top-1] - stack[top];
        } // "-"
      else if (scanok && (scanok = (scanf (" *%n",&i) != EOF)) && (i >= 1))
        {
          top--;
          if (top <= 0)
            {
              printf ("ERROR: Stack underflow\n");
              return EXIT_FAILURE;
            }
          stack[top-1] = stack[top-1] * stack[top];
        } // "*"
      else if (scanok && (scanok = (scanf (" /%n",&i) != EOF)) && (i >= 1))
        {
          top--;
          if (top <= 0)
            {
              printf ("ERROR: Stack underflow\n");
              return EXIT_FAILURE;
            }
          stack[top-1] = stack[top-1] / stack[top];
        } // "/"
      else if (scanok && (scanok = (scanf (" negate%n",&i) != EOF)) && (i >= 1))
        {
          if (top <= 0)
            {
              printf ("ERROR: Stack underflow\n");
              return EXIT_FAILURE;
            }
          stack[top-1] = -stack[top-1];
        }
      else if (scanok && (scanf(" %d", &i) == 1))
        {
          if (top >= STACK_CAPACITY)
            {
              printf ("ERROR!  Stack overflow\n");
              return EXIT_FAILURE;
            }
          stack[top] = i;
          top++;
        }
      else
        {
          if (! scanok)
            {
              done = 1;
              break;
            }
          ch = getchar ();
          if (ch == EOF)
            done = 1;
          else if ((ch == '\n') ||  (ch == ' ') || (ch == '\n'))
            ; // Skip whitespace
          else
            printf ("Invalid input: '%c'\n", ch);
        }
    } // while

  return EXIT_SUCCESS;
} // main