How to calculate the difference in yellow cards

* Example generated by -dataex-. For more info, type help dataex
clear
input byte giornate str10(squadra sfid2) byte ammoniti wanted
2 "Atalanta"   "Juventus"   2 1
2 "Bologna"    "Milan"      1
2 "Brescia"    "Lecce"      2
2 "Cagliari"   "Fiorentina" 1
2 "Chievo"     "Livorno"    3
2 "Fiorentina" "Cagliari"   1
2 "Inter"      "Palermo"    3
2 "Juventus"   "Atalanta"   1 -1
2 "Lazio"      "Reggina"    1
2 "Lecce"      "Brescia"    4
2 "Livorno"    "Chievo"     3
2 "Messina"    "Roma"       5
2 "Milan"      "Bologna"    2
2 "Palermo"    "Inter"      1
2 "Parma"      "Udinese"    3
2 "Reggina"    "Lazio"      3
2 "Roma"       "Messina"    3
2 "Sampdoria"  "Siena"      3
2 "Siena"      "Sampdoria"  2
2 "Udinese"    "Parma"      2
end

gen diffy=0
levelsof squadra, local(sq)
levelsof sfid2, local(sf)
foreach s of local sq{
foreach f of local sf{
replace diffy=ammoniti-ammoniti if "`s'"=="`f'"
}
}