#include<stdio.h>
#include<ctype.h>
#include<string.h>
#define max 25
char stack[max][max];
int top =-1;
void push(char ch[]){
    strcpy(stack[++top],ch);
}
char *pop(){
    return stack[top--];
}
int isoperator(char ch){
    return (ch=='+'||ch=='-'||ch=='*'||ch=='/');
}
int isvalid(char ch){
    return (isalnum(ch)||ch=='+'||ch=='-'||ch=='*'||ch=='/'||ch=='('||ch==')');
}
void fun(char *str){
    int len=strlen(str),ind;
    for(ind=len-1;ind>=0;ind--){
        char ch=str [ind];
        if(!isvalid(ch)){
            printf("Invalid input");
            return ;
        }
        if(!isalnum(ch)){
            char temp[2];
            temp[0]=ch;
            temp[1] ='\0';
            push(temp);
        }
else if(isoperator(ch)){
    char opd1[max],opd2[max],res[max];
    strcpy(opd1,pop());
    strcpy(opd2,pop());
    strcpy(res,opd1);
    strcpy(res,opd2);
    int l=strlen(res);
    res[l]=ch;
    res[l+1]='\0';
    push(res);
}
}
printf("%s",stack[top]);
}
int main(){
    char str[max];
    scanf("%s",max);
    fun(str);
    return 0;
    
}