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

#define STACKSIZE 250

typedef struct{
        int top;
        long long int items[STACKSIZE];
        }STACK;


void push(long long int x, STACK *ps);
int pop(STACK *ps);
int ispositivedigit(char* input);
int isnegativedigit(char* input); //function for detection negative numbers
void printtop(STACK *ps); //bonus


int main()
{
    char input[1024]; //array for loaded values

    int end = 1; //detects end of input
    int count = 0; //variable for detection if no arguments were enetred
    STACK ps = {0, {0}};
    long long int number, a, b, result, printresult;


    while (end != EOF) {

        end = scanf("%s", input);
        count += 1;
        if(end == EOF) {
            count -= 1;
            break;
        }

        //if it's a positive numebr
        if (ispositivedigit(input) == 1) {
            number = atoll(input); //converts string to long long int
            push(number, &ps); //add a number to the top of the stack
        }
        else {
            //string is a negative number or invalid input
            if (strlen(input) > 1) {
                if (isnegativedigit(input) == 1) {
                    number = atoll(input);
                    push(number, &ps);
                }
                else {
                    fprintf(stderr, "Error: Invalid input!\n");
                    exit(EXIT_FAILURE);
                }
            }


            if (strlen(input) == 1) {
                if (strcmp(input, "p") == 0) {
                    printtop(&ps);
                }
                else {

                    a=pop(&ps);
                    b=pop(&ps);

                    if (strcmp(input, "+") == 0) {
                        result = a + b;
                    }

                    else if (strcmp(input, "-") == 0)
                        result = b - a;

                    else if (strcmp(input, "*") == 0)
                        result = a * b;

                    else if (strcmp(input, "/") == 0) {
                        if (a == 0) {
                            fprintf(stderr, "Error: Division by zero!\n");
                            exit(EXIT_FAILURE);
                        }
                        else result = b / a;
                    }

                    else {
                        fprintf(stderr, "Error: Invalid input!\n");
                        exit(EXIT_FAILURE);
                    }

                    push(result, &ps);
                }
            }
        }
    }

    //returns error when no arguments are entered
    if (count == 0) {
        fprintf(stderr, "Error: No arguments!\n");
        exit(EXIT_FAILURE);
    }


    //if the last character isn't "p", the program print the result clasically
    if (strcmp(input, "p") != 0) {
        printresult = pop(&ps);
        printf("%lld\n", printresult);
    }

    return 0;
}


void push(long long int x, STACK *ps)
{
    if(ps->top == STACKSIZE-1){
        fprintf(stderr, "Error: Stack overflow!\n");
        exit(EXIT_FAILURE);
    } else
        ps->items[++(ps->top)] = x;

    return;
}


int pop(STACK *ps)
{
    if(ps->top == 0) {
        fprintf(stderr, "Error: Stack underflow!\n");
        exit(EXIT_FAILURE);
    }
    else
        return(ps->items[(ps->top)--]);
}


int ispositivedigit(char* input)
{
    //must be initialized to 0 after each cycle
    int alldigits = 0;

    //cycle which determines if each character from input is a number or not
    for (unsigned int i=0; i<strlen(input); i++) {
        if (isdigit(input[i]) == 0)
            alldigits = 1; //set alldigits to 1 if the character isn't a number
            return 0;
        }
    return 1;
}


int isnegativedigit(char* input)
{
    if (strncmp(input,"-",1) == 0) {
        for (unsigned int i=1; i<strlen(input); i++) {
            if (isdigit(input[i]) == 0) {
                return 0; //if a character different from number appears, function returns 0
            }
        }
    }
    else
        return 0; //if the firts character isn't "-", function returns 0

    return 1; //it is a negative number
}


void printtop(STACK *ps) //bonus
{
    //not empty stuck
    if (ps->top != 0)
        printf("%lld \n", ps->items[ps->top]);
    //empty stack
    else {
        fprintf(stderr, "Error: Attempt to call for an empty stack!\n");
        exit(EXIT_FAILURE);

    }
}