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

#include <stdlib.h>

#define MAX 1000

typedef struct {
    char *arr[MAX];
    int top;
} Stack;

void init(Stack *s) {
    s->top = -1;
}

int isEmpty(Stack *s){
    return s->top == -1;
}

void push(Stack *s ,char *val) {
    s->arr[++(s->top)] = val;
}

char *pop(Stack *s) {
    if (isEmpty(s)) return NULL;
    return s->arr[(s->top)--];
}

int isOperator(char c) {
    return (c == '+' || c == '-' || c == '*' || c == '/');
}

int validParentheses(char *expr) {
    int count = 0;
    for (int i = 0; expr[i]; i++) {
        if (expr[i] == '(') count++;
        if (count < 0) return 0;
    }
    return count == 0;
}
int validCharacters(char *expr) {
    for(int i = 0; expr[i]; i++){
        if(!isalnum(expr[i]) && !isOpreator(expr[i]) && expr[i] != '(' && expr[i] != ')' && expr[i] !=' ')
        return 0;
    }
    return 1;
}

int main() {
    char expr[MAX];
    fgets(expr, MAX, stdin);
    expr[strcspn(expr, "\n")] = 0;
    
    if(!validParantheses(expr) || !validCharacters(expr)) {

        printf("Invalid input\n");
        return 0;
    }
    
    Stack stack;
    init(&stack);
    
    int len = strlen(expr);
    for (int i = len - 1; i >=0 ; i--){
        if (expr[i] == ' ') continue;
        
        if(isalnum(expr[i])) {
            char *operand =(char *)malloc(2);
            operand[0] = expr[i];
            operand[1] = '\0';
            push(&stack , operand);
        }
        else if (isOperator(expr[i])) {
            char *op1 = pop(&stack);
            char *op2 = pop(&stack);
            
            if (!op1 || !op2) {
                printf("Invalid input\n");
                return 0;
            }
            char *newexpr  = (char *)malloc(strlen(op1) + strlen(op2) +2);
            sprintf(newexpr, "%s%s%c", op1 , op2, expr[i]);
            push(&stack, newexpr);
        }
    }
if(stack.top != 0) {
    printf("Invalid input\n");
    return 0;
}    
printf("%s\n", stack.arr[stack.top]);
return 0;
}