function Main(arg) {
 
    var N = arg - 0;
    
    if(N%2===1) console.log("0");
    
    if (N === 0) console.log("1");

    var ans = BigInt(0);
    var i = 1;
    while (1) {
        var tmp = Math.floor(N / Math.pow(5, i++));
        if (tmp === 0) break;
        ans += BigInt(tmp);
    }

    ans = ans / BigInt(2) - BigInt(2);

        
    console.log(String(ans));

    return;
}

Main(require("fs").readFileSync("/dev/stdin", "utf8"));