const express =require('express')
const app = express()
const ffmpeg = require('fluent-ffmpeg')
const ffmpegPath = require('@ffmpeg-installer/ffmpeg')
const ffprobe = require('@ffprobe-installer/ffprobe')

const first = './videos/first.mp4'
const second = './videos/second.mp4'
const third = './videos/third.mp4'
const fourth = './videos/fourth.mp4'

ffmpeg.setFfprobePath(ffprobe.path)
ffmpeg.setFfmpegPath(ffmpegPath.path)

app.use(express.json())

app.use("/",(req,res)=>{
    res.send("hello")

    ffmpeg()
    .input(first)
    .input(second)
 
    .on('end', function() {
      console.log('files have been merged succesfully');
    })
    .on('error', function(err) {
      console.log('an error happened: ' + err.message);
    })
    .mergeToFile("final.mp4")
  })
  

app.listen(8800,()=>{
    console.log("backend is running 8800")
})

but it shows error whenever we try to merge videos second time

an error happened: ffmpeg exited with code 1: Cannot find a matching stream for unlabeled input pad 3 on filter Parsed_concat_0

an error happened: ffmpeg exited with code 1: Cannot find a matching stream for unlabeled input pad 3 on filter Parsed_concat_0