#include <stdio.h>
#include <stdlib.h>
 
/* constants should be centralized */
#define PESO 13.170025
#define USD  0.07593
 
void clear(FILE *strm)
{
    int c;
 
    do c = getchar(); while (c != '\n' && c != EOF);
}
 
int main()
{
    /* double is usually a better choice than float */
    double amount;
    int choice;
 
    /* long strings can be broken down to avoid multiple printfs */
    /* if you don't print '\n', flush the stream */
    printf("1) USD to PESO\n"
           "2) PESO to USD\n"
           "> ");
    fflush(stdout);
 
    /* check input for failure */
    if (scanf("%d", &choice) != 1)
    {
        fputs("Bad input\n", stderr);
        return EXIT_FAILURE;
    }
 
    /* consolidate code to avoid redundancy */
    /* if you don't print '\n', flush the stream */
    printf("Enter the amount: ");
    fflush(stdout);
 
    /* check input for failure */
    if (scanf("%lf", &amount) != 1)
    {
        fputs("Bad input\n", stderr);
        return EXIT_FAILURE;
    }
 
    if (choice == 1)
    {
        printf("%gUSD == %gPESO\n", amount, amount * PESO);
    }
    else
    {
        printf("%gPESO == %gUSD\n", amount, amount * USD);
    }
 
    clear(stdin);
    fputs("Press <Enter> to continue...", stderr);
    getchar();
 
    return EXIT_SUCCESS;
}