>>171
int main( void )
{
int wantBuy = 0;
int needBuy = 0;
int serviceTotal = 0;
int serviceEach = 0;

scanf( "%d", &wantBuy );

serviceTotal = wantBuy / 5;
do{
serviceEach = serviceTotal / 5
serviceTotal += serviceEach;
}while( serviceEach != 0 );
needBuy = wantBuy ｰ serviceTotal;

return 0;
}

ケータイからなので動くか分からん(´・ω・｀)