error: ‘f’ cannot be used as a function y=f(n); 求解

#include <stdio.h>

long f(int n)
    {
    long p;
    if(n==0||n==1)
    p=1;
    else p=f(n-1)*n;
    return p;
    }
int main(void)
{
    int n;
    long y,f(n);
    printf("please input a number:");
    scanf("%d",&n);
    while(n<0)
    {
        printf("error,input again:");
        scanf("%d",&n);
        }
        y=f(n);
        printf("%d!=%ld\n",n,y);
        return 0;
    }