How can I concatenate data in crystal reports 2010?

Good morning.

I was asked to print the number of years and months that a client has been residing in some state, for this I must add the words "years" and "months" or "year" and "month" depending on the case.

It's the first time I use Crystal Reports, so I'm not sure if I did this well, I wrote the following code and it gives me an error that says "the string is not numeric", I appreciate your help.

if((tonumber({CRE050.ANIOS_RESIDENCIA})>1) and (tonumber({CRE050.MESES_RESIDENCIA})>1))

then({CRE050.ANIOS_RESIDENCIA} & "Años"  & {CRE050.MESES_RESIDENCIA} & "Meses";)

else 

if((tonumber({CRE050.ANIOS_RESIDENCIA})>1)and (tonumber({CRE050.MESES_RESIDENCIA})=1))

then ({CRE050.ANIOS_RESIDENCIA} & "Años"  & {CRE050.MESES_RESIDENCIA} & "Mes";)

else

if((tonumber({CRE050.ANIOS_RESIDENCIA})=1)and(tonumber({CRE050.MESES_RESIDENCIA})>1))

then({CRE050.ANIOS_RESIDENCIA} & "Año"  & {CRE050.MESES_RESIDENCIA} & "Meses";)

else

if((tonumber({CRE050.ANIOS_RESIDENCIA})=1)and(tonumber({CRE050.MESES_RESIDENCIA})=1))

then({CRE050.ANIOS_RESIDENCIA} & "Año"  & {CRE050.MESES_RESIDENCIA} & "Meses";)

else

if((tonumber({CRE050.ANIOS_RESIDENCIA})=0)and(tonumber({CRE050.MESES_RESIDENCIA})=1))

then({CRE050.MESES_RESIDENCIA} & "Mes";)

else

if((tonumber({CRE050.ANIOS_RESIDENCIA})=0)and(tonumber({CRE050.MESES_RESIDENCIA})>1))

then({CRE050.MESES_RESIDENCIA} & "Meses";)

else

if((tonumber({CRE050.ANIOS_RESIDENCIA})=1)and(tonumber({CRE050.MESES_RESIDENCIA})=0))

then({CRE050.ANIOS_RESIDENCIA} & "Año";)

else

if((tonumber({CRE050.ANIOS_RESIDENCIA})>1)and(tonumber({CRE050.MESES_RESIDENCIA})=0))

then({CRE050.ANIOS_RESIDENCIA} & "Años";)