#include <stdio.h>

#include <stdlib.h>

struct Stack {

    int capacity;

    int top;

    int* array;

};

struct Stack* createStack(unsigned capacity) {

    struct Stack* stack = (struct Stack*)malloc(sizeof(struct Stack));

    stack->capacity = capacity;

    stack->top = -1;

    stack->array = (int*)malloc(stack->capacity * sizeof(int));

    return stack;

}

int isEmpty(struct Stack* stack) {

    return stack->top == -1;

}

int isFull(struct Stack* stack) {

    return stack->top == stack->capacity - 1;

}

void push(struct Stack* stack, int item) {

    if (isFull(stack)) {

        return;

    }

    stack->array[++stack->top] = item;

}

int pop(struct Stack* stack) {

    if (isEmpty(stack)) {

        return -1;

    }

    return stack->array[stack->top--];

}

long long int factorial(int n) {

    if (n <= 1) {

        return 1;

    }

    struct Stack* stack = createStack(n);

    long long int result = 1;

    while (n > 1) {

        push(stack, n);

        n--;

    }

    while (!isEmpty(stack)) {

        result *= pop(stack);

    }

    free(stack);

    return result;

}

int main(void) {

    int num;

    printf("insert num: ");

    scanf("%d", &num);

    if (num < 0) {

        printf("negative cant be calculated.\n");

    } else {

        long long int result = factorial(num);

        printf("factorial(%d): %lld\n", num, result);

    }

    return 0;

}