#include <stdio.h>
#include <string.h>
#include <ctype.h>

char stack[100][100];
int top = -1;

void push(char str[]) {
    strcpy(stack[++top], str);
}

char* pop() {
    return stack[top--];
}

int isOperator(char c) {
    return (c == '+' || c == '-' || c == '*' || c == '/' || c == '^');
}

int main() {
    char prefix[100];
    scanf("%[^\n]", prefix);

    int len = strlen(prefix);

    for (int i = len - 1; i >= 0; i--) {
        char c = prefix[i];

        if (c == ' ')
            continue;

        if (isalnum(c)) {
            char temp[2] = {c, '\0'};
            push(temp);
        }
        else if (isOperator(c)) {
            if (top < 1) {
                printf("Invalid input");
                return 0;
            }

            char op1[100], op2[100], expr[100];
            strcpy(op1, pop());
            strcpy(op2, pop());

            sprintf(expr, "%s%s%c", op1, op2, c);
            push(expr);
        }
        else {
            printf("Invalid input");
            return 0;
        }
    }

    if (top != 0) {
        printf("Invalid input");
        return 0;
    }

    printf("%s", pop());
    return 0;
}