fun main() {
    val data = List(readInt()) { readln() }
    val before = Array(26) { BooleanArray(26) }
    for ((l, r) in data.zipWithNext()) {
        if (l.startsWith(r) && l != r) {
            println("impossible")
            return
        }
        l.indices.firstOrNull { l[it] != r[it] }?.let {
            before[l[it].code - 'a'.code][r[it].code - 'a'.code] = true
        }
    }
    val deg = IntArray(26) { v -> before.count { it[v] }}
    println(buildString {
        repeat(26) {
            val v = deg.indexOf(0)
            if (v == -1) {
                println("impossible")
                return
            }
            append(('a'.code + v).toChar())
            deg[v] = 1000
            for (i in deg.indices) {
                if (before[v][i]) {
                    deg[i]--
                }
            }
        }
    })
}

private fun readInt() = readln().toInt()
private fun readStrings() = readln().split(" ")