5 ONERRORGOTO900
20 MODE8:DIM:a=0:b=0:c$=CHR$(5)
30 CLS:PRINT"ćab           a:b:radius";
40 s$="a":z=a:GOSUB500:a=z
50 s$="b":z=b:GOSUB500:b=z
60 LOCATE0,1:PRINTc$;"Area =";PI*a*b;:z$=INPUT$(1,@):GOTO40
500 LOCATE0,1:PRINTc$;s$;"=";z;"?";:INPUT@42;z:LOCATE0,0:IFz<0 THEN500 ELSERETURN
900 IFERR=1 THENCLS:ONERRORGOTO0
910 IFERL=500 THENRESUME500
920 LOCATE0,1:PRINTc$;"not found";:z$=INPUT$(1,@):RESUME30