I have an input file
12.4 1.72849432773174e+01 -7.74784188610632e+01
12.5 9.59432114416327e-01 -7.87018212757537e+01
15.6 5.20139995965960e-01 -5.61612429666624e+01
29.3 3.76696387248366e+00 -7.42896194101892e+01
32.1 1.86899877018077e+01 -7.56508762501408e+01
35 6.98857157014640e+00 -6.47719177518354e+01
57.5 9.93224752487084e+00 -5.98507250538671e+01
59.5 1.17880137446671e+01 -6.70358856818233e+01
59.6 3.39353476095376e+00 -5.85719268969042e+01
64.6 1.39100633884576e+01 -7.89068504596848e+01
66.1 3.29754641066104e+00 -6.11126946124472e+01
68.3 1.34614891224838e+01 -7.96561570561526e+01
75.2 1.27732250188725e+00 -7.98537412569579e+01
75.7 1.81103716761368e+01 -7.96494146285998e+01
79.6 6.16383337522536e+00 -6.29030439858399e+01
92.8 1.52331049249522e+01 -6.51748608352182e+01
94.6 7.26150084491214e+00 -6.93621024271574e+01
100.1 1.68539103307157e+01 -7.85748993775416e+01
100.9 3.52929142412477e+00 -6.86640757476861e+01
101 1.62464294145481e+00 -6.12247635460096e+01
101.1 1.67667000571671e+01 -7.60963191396523e+01
109.1 1.89539766434845e+01 -3.22357460422950e+01
124.2 5.77306017328185e+00 -7.74816073271627e+01
124.3 1.62709216276807e-01 -6.38710866726218e+01
124.4 1.62418063967260e+00 -5.06164124324214e+01
124.5 1.15920020337606e+00 -5.39562951251459e+01
127.5 1.63638664576208e+01 -7.53341787458119e+01
I want to grep only lines that start with an integer number,
any recommendations?