음 되다

you just have to X

This is -(으)면 되다 'it's enough to just X' squeezed shorter in fast speech, so 있음 되지 is 있으면 되지. It waves a problem away as easily solved: 너는 그냥 가만히 있음 되지 'you just have to sit still', 저녁 많이 먹음 되지 뭐 'I'll just eat a big dinner, no big deal'. Every line here is casual, and the 뭐 tacked on the end adds the shrug. Do not write it this way.

Register: casual speech only