#include<stdio.h>
#include<string.h>
#include<ctype.h>
#define MAX 100
char stack[MAX][MAX];
int top=-1;
void push(char st[]){
    strcpy(stack[++top], str);
}
char* pop(){
    return stack[top--];
}
int isOperator(char ch){
    return (ch=='+'||ch=='-'||ch=='*'||ch=='/');
}
int main(){
    char prefix[MAX];
    scanf("%s",prefix);
    int len=strlen(pefix);
    for(int i=len;i>0;i--){
        char ch=prefix[i];
        if(isalnum(ch)){
            char temp[2]={ch,'\0'};
            push (temp);
        }
        else if(isOperator(ch)){
            if(top<1){
                printf("Invalid input");
                return 0;
            }
            char op1[MAX],op2[MAX],exp[MAX];
            strcpy(op1, pop());
            strcpy(op2, pop());
            strcpy(exp, op1);
            strcat(exp, op2);
            int l=strlen(exp);
            exp[l]=ch;
            exp[l+1]='\0';
            push(exp);
        }else{
            printf("Invalid input");
            return 0;
        }
    }
    if(top!=0){
        printf("Invalid input");
        return 0;
    }
    printf("%s ",stack[top]);
    return 0;
}