C++ convert between kilograms and pounds

/*
    kg_lb.cpp
    g++ -o kg_lb kg_lb.cpp
    kg_lb p 20 25

    https://www.metric-conversions.org/weight/kilograms-to-pounds.htm
    1kg = 2.20462lb
    1lb = 0.45359kg
*/

#include <iostream>
#include <string>
#include <cstdlib>

using namespace std;

const char* message = "kg_lb\nusage: kg_lb x y\nwhere x equals k or p, for kilograms or pounds, and y equals the weight number.\nexample to convert 30 lb to kg: kg_lb p 30\nsupports multiple weight inputs";
const char* unit[] = {"kilograms (kg)", "pounds (lb)"};

int main(int argc, char** argv)
{
    float input,output;
    bool which;
    if (argc < 3)
    {
        cout << message << endl;
        return 0;
    }
    if (argv[1][0]=='k') which=0;
    else if (argv[1][0]=='p') which=1;
    else
    {
        cout << "first argument is invalid - " << argv[1] << endl << message << endl;
        return 0;
    }
    for (int i=2; i<argc; i++)
    {
        input = atof(argv[i]);
        if (which) output = input * 0.45359;
        else output = input * 2.20462;
        cout << input << " " << unit[which] << " = " << output << " " << unit[!which] << endl;
    }
    return 0;
}