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