\documentclass[a4paper,12pt]{article}
\usepackage[OT1]{fontenc}
\usepackage{txfonts}
\begin{document}
\[\fint\frac{dz}{z-1}\]
\end{document}
\documentclass[a4paper,12pt]{article}
\usepackage[OT1]{fontenc}
\usepackage{esint}
\begin{document}
\[\fint\frac{dz}{z-1}\]
\end{document}