import core.bitop, std.algorithm, std.ascii, std.bigint, std.conv, std.math,
    std.functional, std.numeric, std.range, std.stdio, std.string, std.random,
    std.typecons, std.container, std.format;
static import std.ascii;

// dfmt off
T lread(T = long)(){return readln.chomp.to!T();}
T[] lreads(T = long)(long n){return generate(()=>readln.chomp.to!T()).take(n).array();}
T[] aryread(T = long)(){return readln.split.to!(T[])();}
void scan(TList...)(ref TList Args){auto line = readln.split();
foreach (i, T; TList){T val = line[i].to!(T);Args[i] = val;}}
alias sread = () => readln.chomp();enum MOD = 10 ^^ 9 + 7;
alias PQueue(T, alias less = "a<b") = BinaryHeap!(Array!T, less);
// dfmt on

void main()
{
    long N = lread();
    auto cnt = new long[][](2, 30);
    cnt[1][] = long.max;
    foreach (i; 0 .. N)
    {
        auto S = sread();
        cnt[i & 1][] = 0;
        foreach (c; S)
            cnt[i & 1][c - 'a']++;
        // writeln(cnt[i & 1]);
        foreach (j; 0 .. 30)
            cnt[i & 1][j] = cnt[(i + 1) & 1][j].min(cnt[i & 1][j]);
    }
    // writeln(cnt[(N + 1) & 1]);
    string ans;
    foreach (i; 0 .. 30)
    {
        ans ~= repeat(cast(char)('a' + i)).take(cnt[(N + 1) & 1][i]).array;
    }
    writeln(ans);
}
