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\documentclass[12pt,a4paper,twoside=semi]{scrreprt}
\usepackage{blindtext}
\usepackage[T1]{fontenc}
\usepackage[utf8]{inputenc}
\usepackage{lmodern}
\usepackage[ngerman]{babel}
\usepackage{fancyhdr}
 \pagestyle{fancy}
 \fancyhf{}
 \fancyhead[LE]{\slshape \nouppercase{\leftmark}} % Links oben auf geraden Seiten
 \fancyhead[LO]{\slshape \nouppercase{\rightmark}} % Links oben auf ungeraden Seiten
 \fancyhead[R]{\bfseries \thepage}  % Rechts oben
 \renewcommand{\headrulewidth}{1pt}
 \renewcommand{\footrulewidth}{0pt} 

 \renewcommand{\chaptermark}[1]{%
 \markboth{\thechapter\ \ \ \ #1}{}}

 \renewcommand{\sectionmark}[1]{%
 \markright{\thesection\ \ \ \ #1}{}}

 \topmargin10mm
 \addtolength{\headheight}{2pt}
 \footskip10mm
 \oddsidemargin10mm
 \evensidemargin10mm
 \textwidth150mm
 \textheight215mm
 \marginparsep10mm
 \marginparwidth25mm
 \newlength{\fullwidth}
 \setlength{\fullwidth}{\textwidth}
 \setlength{\headwidth}{\fullwidth}
\begin{document}
\Blinddocument
\end{document}