#include <stdio.h>
int main(void)
{
//freopen("input.txt","r",stdin);
//freopen("output.txt","w",stdout);
int N,M,A,B,minImporto,ImportoTotA,Importo,Mbis,c;
scanf("%d",&N
);//valorizzo il numero di giorni di validità del contratto scanf("%d",&M
);//valorizzo il numero di giorni di tasse pagate nel secondo modo scanf("%d",&A
);//valorizzo il costo in euro per 1 giorno di tasse scanf("%d",&B
);// valorizzo il costo in euro per M giorni di tasse c=2;
ImportoTotA=(N * A);
if(ImportoTotA<=B)
minImporto=ImportoTotA;
else//ImportoTotA>B
{
if(M>=N)
minImporto=B;
else//M<N
{
minImporto=B+(A * (N-M));//aggiungo all'importo B il costo di un giorno di tasse moltiplicato per i giorni rimanenti
printf("minImporto=%d\n",minImporto
); Mbis=M;
while(Mbis<N)//valuto i multipli del costo per M giorni di tasse
{
Mbis=(Mbis * c);
Importo=(B * c);
c++;
}
printf("Importo=%d\n",Importo
); if(Importo<=minImporto)
minImporto=Importo;
}
}
printf("%d",minImporto
);//minimo importo di tasse da pagare return 0;
}
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