/*
 * interest - interest computation program.
 *
 * Usage: interest nom comp-den comp-num principal
 *
 *	nom
 *		nominal interest rate, in percent
 *	comp-den, comp-num
 *		denominator and numerator of the compounding fraction
 *		to be applied to the interest rate
 *	principal
 *		principal amount to which the interest is to be applied
 *
 * For example, to use this to work out two months' interest, if the
 *  rate is 4% per half-year ("8% per year compounded semi-annually"),
 *  and interest really is done monthly, and the principal is
 *  $36000.00,
 *	interest 4 6 2 3600000
 *  will print the interest, rounded to the nearest cent.
 *
 * As another example, the same calculation but where interest is
 *  calculated daily and the two months in question are June and July
 *  of a non-leap-year (61 days out of 365/2=182.5),
 *	interest 4 182.5 61 3600000
 *  would be appropriate.
 *
 * The "principal" value must always be an integer; the other three
 *  values may be floating-point.  The result is rounded to the nearest
 *  integer for printing.
 */

#include <math.h>
#include <stdio.h>
#include <stdlib.h>

extern const char *__progname;

static double nom;
static double comp_den;
static double comp_num;
static int principal;

static void usage(void)
{
 fprintf(stderr,"%s: usage: %s interest-rate comp-den comp-num principal\n",__progname,__progname);
}

static int intarg(const char *arg, int *vp, const char *tag)
{
 char *cp;

 *vp = strtol(arg,&cp,0);
 if ((cp == arg) || *cp)
  { fprintf(stderr,"%s: invalid %s value `%s'\n",__progname,tag,arg);
    return(1);
  }
 return(0);
}

static int floatarg(const char *arg, double *vp, const char *tag)
{
 char *cp;

 *vp = strtod(arg,&cp);
 if ((cp == arg) || *cp)
  { fprintf(stderr,"%s: invalid %s value `%s'\n",__progname,tag,arg);
    return(1);
  }
 return(0);
}

static void handleargs(int ac, char **av)
{
 int skip;
 int errs;
 int argno;

 skip = 0;
 errs = 0;
 argno = 0;
 for (ac--,av++;ac;ac--,av++)
  { if (skip > 0)
     { skip --;
       continue;
     }
    if (**av != '-')
     { switch (argno++)
	{ case 0: errs += floatarg(*av,&nom,      "interest rate"          ); break;
	  case 1: errs += floatarg(*av,&comp_den, "compounding denominator"); break;
	  case 2: errs += floatarg(*av,&comp_num, "compounding numerator"  ); break;
	  case 3: errs += intarg  (*av,&principal,"principal"              ); break;
	  default:
	     fprintf(stderr,"%s: extra argument `%s'\n",__progname,*av);
	     errs ++;
	     break;
	}
       continue;
     }
#if 0
    if (0)
     {
needarg:;
       fprintf(stderr,"%s: %s needs a following argument\n",__progname,*av);
       errs ++;
       continue;
     }
#define WANTARG() do { if (++skip >= ac) goto needarg; } while (0)
    if (!strcmp(*av,"-display"))
     { WANTARG();
       displayname = av[skip];
       continue;
     }
    if (!strcmp(*av,"-sync"))
     { sync = 1;
       continue;
     }
#undef WANTARG
#endif
    fprintf(stderr,"%s: unrecognized option `%s'\n",__progname,*av);
    errs ++;
  }
 if (argno < 4)
  { fprintf(stderr,"%s: not enough arguments\n",__progname);
    errs ++;
  }
 if (errs)
  { usage();
    exit(1);
  }
}

static int roundint(double d)
{
 int i;
 double f;

 i = d;
 f = d - i;
 if (f < -.5) i--; else if (f > .5) i ++;
 return(i);
}

int main(int, char **);
int main(int ac, char **av)
{
 handleargs(ac,av);
 printf("%d\n",roundint(expm1((log1p(nom/100)*comp_num)/comp_den)*principal));
 exit(0);
}