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

#define STACK_SIZE 20

void push(int* stack, int* top, int element);
int pop(int* stack, int* top);
bool is_empty(int* top);
bool is_full(int* top);
void stack_overflow(void);
void stack_underflow(void);
int evaluate(int i, int j, int k);

int main(void){
    int top1=0,top2=0;
  int *top_operators=&top1;
  int *top_operands=&top2;
  int stack_operands[STACK_SIZE];
  int stack_operators[STACK_SIZE];
  int *operands = &stack_operands[0];
  int *operators = &stack_operators[0];
  char next;
  int temp, temp1, result;

  scanf("%c", &next);
  while(next!='='){
    if(next=='+' || next=='-' || next=='*' || next=='/')
    {
        push(operators,top_operators,next);
        printf("first");
    }

    else if(isdigit(next)){
        printf("a");
      //temp=next-'0';
      push(operands,top_operands,next);
      printf("%d\n", pop(operands,top_operands));
      push(operands,top_operands,next);
    }
    else if(next==')'){
        printf("b");
      temp=pop(operands,top_operands);
      temp1=pop(operands,top_operands);
      result=evaluate(temp,temp1,pop(operators,top_operators));
      push(operands,top_operands,result);

    }
    else if(next!='('){
        printf("c");
      printf("Invalid input.");
      exit(EXIT_FAILURE);
    }
    scanf("%c",&next);
  }
  printf("hi");
  //result=pop(operands,top_operands);
  if(!is_empty(top_operands) || !is_empty(top_operators)){
      printf("d");
    printf("Invalid input.");
    exit(EXIT_FAILURE);

  }
  else{
      printf("boo");
    printf("%d\n",result);
    printf("e");
    return(0);
  }
}

void push(int* stack, int* top, int element){
  if(is_full(top))
    stack_overflow();
  else
  {
      *(stack+(*top++))=element;
      printf("q");
  }

}

int pop(int* stack, int* top){
  if(is_empty(top))
     stack_underflow();
  else
    return *(stack+(--*top));
}

bool is_empty(int* top){
  return *top==0;
}

bool is_full(int* top){
  return *top == STACK_SIZE;
}

void stack_overflow(void){
  printf("Error. Stack overflow.\n");
  exit(EXIT_FAILURE);
}

void stack_underflow(void){
  printf("Error. Stack underflow.\n");
  exit(EXIT_FAILURE);
}

 int evaluate(int i, int j, int k){
   int result;
   if(k=='+')
     result=(i+j);
   else if(k=='-')
     result=(i-j);
   else if(k=='*')
     result=(i*j);
   else
     result=(i/j);
   return result;
 }