function Main(input) {

    var data = input.split("\n").filter(s => s.length > 0)
    var n = data.shift() * 1
let r = ''

    function findMinIdx(input){
    const codes = input.map(s=>s.slice(0, 1)).map(s=>[].reduce.call(s, (sum ,c)=> sum += c.charCodeAt(0), 0))
    const minCode = Math.min(...codes)
    const minIdxes = codes.map((c, i) => [c, i]).filter(a => a[0] == minCode).map(a => a[1])
    if (minIdxes.length == 1) {
        return minIdxes.pop()
    }
    let nextInputIndex = []
    let nextInput = input.filter((s, i) => {
        let isNext = s.charCodeAt(0) == minCode
        if (isNext){
            nextInputIndex.push(i)
        }
        return isNext
    }).map(s => s.slice(1))
    if (nextInput.length < 1) {
        return minIdxes.pop()
    }
    return nextInputIndex[findMinIdx(nextInput)]
}
var input = data
while (input.length > 0) {
    
    const minIdx = findMinIdx(input) 
    const str = input[minIdx]
    r += str[0]
    input[minIdx] = str.slice(1)
    input = input.filter(s => s.length > 0)
}
console.log(r)
}

// Don't edit this line!
Main(require("fs").readFileSync("/dev/stdin", "utf8"));