#include<stdio.h>
#include<ctype.h>
#include<string.h>
#define MAX 25

char stack[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=='/'); )
}

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 op1[MAX], op2[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;
}