//  [javascript]
process.stdin.resume();
process.stdin.setEncoding('utf8');

var g_input = '';
process.stdin.on('data', function(chunk) {
    g_input += chunk;
});

process.stdin.on('end', function() {
    Main(g_input.split(/\r?\n/));
});

function UInt64(value) {
    var uint64 = Object.create(UInt64.prototype);
    uint64._wordArray = new Array(4);

    uint64.set(value);

    return uint64;
}

UInt64.compare = function(a, b) {
    for (var i = a._wordArray.length; i >= 0; i++) {
        if (a._wordArray[i] > b._wordArray[i]) {
            return 1;
        }
        else if (a._wordArray[i] < b._wordArray[i]) {
            return -1;
        }
        else {
        }
    }
    return 0;
};

UInt64.prototype.set = function(value) {
    var div = 0;

    if (UInt64.prototype.isPrototypeOf(value)) {
        for (var i = 0; i < this._wordArray.length; i++) {
            this._wordArray[i] = value._wordArray[i];
        }
    }
    else if (typeof value == 'number') {
        div = 1;
        for (var i = 0; i < this._wordArray.length; i++) {
            this._wordArray[i] = (Math.floor(value / div) & 0xFFFF);
            div *= 0x10000;
        }
    }
    else {
        for (var i = 0; i < this._wordArray.length; i++) {
            this._wordArray[i] = 0;
        }
    }
};

UInt64.prototype.add = function(value) {
    var prevCarry = 0;
    var carry = 0;

    for (var i = 0; i < this._wordArray.length; i++) {
        carry = (((this._wordArray[i] + value._wordArray[i] + prevCarry) & 0xFFFF0000) >>> 16);
        this._wordArray[i] = ((this._wordArray[i] + value._wordArray[i] + prevCarry) & 0xFFFF);
        prevCarry = carry;
    }

    return this;
};

UInt64.prototype.multiply = function(value) {
    var prevCarry = 0;
    var carry = 0;
    var prod = UInt64(0);

    for (var i = 0; i < this._wordArray.length; i++) {
        prevCarry = 0;
        for (var j = 0; j < value._wordArray.length; j++) {
            if (i + j >= prod._wordArray.length) {
                break;
            }

            carry = (((this._wordArray[i] * value._wordArray[j] + prevCarry + prod._wordArray[i + j]) & 0xFFFF0000) >>> 16);
            prod._wordArray[i + j] = ((this._wordArray[i] * value._wordArray[j] + prevCarry + prod._wordArray[i + j]) & 0xFFFF);
            prevCarry = carry;
        }
    }

    this.set(prod);

    return this;
};

UInt64.prototype.isZero = function() {
    var ret = true;

    for (var i = 0; i < this._wordArray.length; i++) {
        if (this._wordArray[i] > 0) {
            ret = false;
            break;
        }
    }

    return ret;
};

UInt64.prototype.toString = function() {
    var work = UInt64(this);
    var prevMod = 0;
    var mod = 0;
    var str = '';

    if (work.isZero()) {
        str = '0';
    }
    else {
        while (!work.isZero()) {
            mod = 0;
            prevMod = 0;
            for (var i = work._wordArray.length - 1; i >= 0; i--) {
                mod = (work._wordArray[i] + prevMod) % 10;
                work._wordArray[i] = Math.floor((work._wordArray[i] + prevMod) / 10);
                prevMod = mod << 16;
            }

            str = String.fromCharCode('0'.charCodeAt() + mod) + str;
        }
    }

    return str;
};


function Main(input) {

    var n = parseInt(input[0]);
    var ans = UInt64(1);

    var l = [UInt64(2), UInt64(1)];
    for (var i = 2; i <= n; i++) {
        ans.set(l[0]);
        ans.add(l[1]);
        l[0].set(l[1]);
        l[1].set(ans);
    }

    console.log(ans.toString());

    return 0;
}