var assert = {
    test: 1,
    equal: function(exp, target) {
        console.log("[test](%d) expected "+target+" got " + exp + " (%s)", this.test++, (exp == target ? 'SUCCESS' : 'FAIL'));
    }
};

// SKITTLES
// Given a target goal in kilograms, and a number of
// small bags of skittles (1kg) and large bags of skittles (5kg)
// return the number of small bags used to make up the total,
// assuming we'll use as many big bags as possible.
// If not possible, return -1.
//
function createPackage(small, big, goal) {
    var lg = Math.floor(goal / 5),
        sm = Math.floor(goal - lg);
    
    goal = goal - ((big <= lg) ? (big*5) : (lg*5));
    return (small >= goal) ? goal : -1;
}

assert.equal(
 createPackage(4, 1, 9),
 4
);

assert.equal(
 createPackage(4, 1, 10),
 -1
);

assert.equal(
 createPackage(4, 1, 7),
 2
);

assert.equal(
 createPackage(6, 2, 7),
 2
);

assert.equal(
 createPackage(4, 1, 5),
 0
);

assert.equal(
 createPackage(4, 1, 4),
 4
);

assert.equal(
 createPackage(5, 4, 9),
 4
);

assert.equal(
 createPackage(9, 3, 18),
 3
);

console.log('Success!');