#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)
}
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",str);
    fun(str);
    return 0;
}