How do I put a label inside a Feynman diagram circle?

I have this simple Feynman diagram with a circle:

I would like to put the number "1" in the middle of that circle.

I've tried these:

\fmfiv{lab=1}{c}

\fmfiv{lab=1}{.5[nw,se])

\fmfiv{lab=1}{(.4999w, .5h)}

\fmfiv{lab=1}{(.5001w, .5h)}

but whatever constants I put, latex refuses to put it INSIDE the circle.

The latex:

\documentclass[24pt]{article}
\usepackage{amsmath,amsfonts,epsf}
\usepackage{amssymb}
\usepackage[pdftex]{graphicx}
\usepackage{grffile}
\usepackage{feynmp-auto}

\begin{document}

\begin{fmffile}{2ptcorrection1}
\begin{fmfgraph*}(220,12)
\fmfpen{thick}
\fmfleft{i1}
\fmfright{o1}
\fmf{plain}{i1,v1}
\fmf{dashes, left=1, tension=0.3}{v1,v2}
\fmf{plain, right=1, tension=0.3}{v1,v2}
\fmf{plain}{v2,o1}
\end{fmfgraph*}
\end{fmffile}

\end{document}


btw, the coordinates are: "c" for center, sw, se, ne, nw.

Update: thank you hftf for the answer.

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I was able to make the label appear to be in the center of the circle by using a phantom (invisible) edge across the circle and setting the label.dist to 0 to place the label in the middle of that edge.

\documentclass{minimal}
\usepackage{grffile}
\usepackage{feynmp-auto}

\begin{document}

\begin{fmffile}{2ptcorrection1}
\begin{fmfgraph*}(220,12)
\fmfpen{thick}
\fmfleft{i1}
\fmfright{o1}
\fmf{plain}{i1,v1}
\fmf{dashes, left=1, tension=0.3}{v1,v2}
\fmf{plain, right=1, tension=0.3}{v1,v2}
\fmf{plain}{v2,o1}

% This is where the magic happens:
\fmf{phantom,label.dist=0,label=1}{v1,v2}

\end{fmfgraph*}
\end{fmffile}

\end{document}


And this is what it looks like:

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