\documentclass{beamer}
\usetheme{Frankfurt}
\usecolortheme{beaver}
\definecolor{links}{HTML}{2A1B81}
\hypersetup{colorlinks,linkcolor=,urlcolor=links}
\usepackage{hyperref}
\usepackage[utf8]{inputenc} % this is needed for german umlauts
\usepackage[ngerman]{babel} % this is needed for german umlauts
\usepackage[T1]{fontenc}    % this is needed for correct output of umlauts in pdf
\usepackage{braket}         % needed for \Set
\usepackage{csquotes}
\usepackage{enumitem}
\setitemize{label=\usebeamerfont*{itemize item}%
  \usebeamercolor[fg]{itemize item}
  \usebeamertemplate{itemize item}}
\usepackage{amsmath, amssymb}
\usepackage{bm}
\usepackage{dsfont}
\usepackage{nicefrac}
\usefonttheme[onlymath]{serif}
\usepackage{siunitx}        % this package is for units!
\sisetup{range-phrase=--, range-units=single}

\usepackage{booktabs}  % for \toprule, \midrule and \bottomrule
\selectlanguage{ngerman}

\usepackage{tikz}
\usetikzlibrary{arrows,positioning, calc}

\setbeamertemplate{navigation symbols}{}
\addtobeamertemplate{navigation symbols}{}{%
    \usebeamerfont{footline}%
    \usebeamercolor[fg]{footline}%
    \hspace{1em}%
    \insertframenumber/\inserttotalframenumber
}
\makeatletter
\setbeamertemplate{footline}
{%
  \pgfuseshading{beamer@barshade}%
  \ifbeamer@sb@subsection%
    \vskip-9.75ex%
  \else%
    \vskip-7ex%
  \fi%
  \begin{beamercolorbox}[ignorebg,ht=2.25ex,dp=3.75ex]{section in head/foot}
    \insertnavigation{\paperwidth}
  \end{beamercolorbox}%
  \ifbeamer@sb@subsection%
    \begin{beamercolorbox}[ignorebg,ht=2.125ex,dp=1.125ex,%
      leftskip=.3cm,rightskip=.3cm plus1fil]{subsection in head/foot}
      \usebeamerfont{subsection in head/foot}\insertsubsectionhead
    \end{beamercolorbox}%
  \fi%
}%
\setbeamertemplate{headline}{%
% \hskip1em\usebeamercolor[fg]{navigation symbols dimmed}%
% \insertslidenavigationsymbol%
% \insertframenavigationsymbol%
% \insertsectionnavigationsymbol%
% \insertsubsectionnavigationsymbol%
% \insertdocnavigationsymbol%
% \insertbackfindforwardnavigationsymbol%
}
\makeatother

\begin{document}

\title{Interventions}
% \subtitle{A subtitle}
\author{Martin Thoma}
\date{10. August 2015}
\subject{Causality}

\frame{\titlepage}

% Show table of contents
% \frame{
%     \frametitle{Inhalt}
%     \setcounter{tocdepth}{1}
%     \tableofcontents
%     \setcounter{tocdepth}{2}
% }

%\AtBeginSection[]{
%    \InsertToC[sections={\thesection}]  % shows only subsubsections of one subsection
%}

\input{introduction}
\input{main}
\input{end}
\end{document}