uses crt;
var isPrime: array[1..100] of boolean;
    i,j,res,toInt: integer;
    s,t: ansistring;
begin
    for i := 1 to 100 do isPrime[i] := true;
    isPrime[1] := false;
    isPrime[2] := true;
    for i := 3 to 100 do
        for j := 2 to i - 1 do
            if (i mod j) = 0 then begin
                isPrime[i] := false;
                break;
            end;
    readln(s);
    if length(s) > 200 then
    begin
        write(-1);
        halt;
    end;
    res := -1;
    t := '';
    for i := 1 to length(s) do
        if (ord(s[i]) - 48 >= 0) and (ord(s[i]) - 48 <= 9) then
            for j := i to length(s) do
                if (ord(s[j]) - 48 >= 0) and (ord(s[j]) - 48 <= 9) and (length(t) <= 1) then begin
                    t := t + s[j];
                    if length(t) = 1 then toInt := ord(t[1]) - 48
                    else toInt := (ord(t[1]) - 48)*10 + (ord(t[2]) - 48);
                    if (isPrime[toInt]) and (toInt > res) then res := toInt;
                end else begin
                    t := '';
                    break;
                end;
    write(res);
end.