function main(input) {
  let [N] = getIntegers(input, " ");
  let L = getIntegers(input, " ");
  let sum = L.reduce((a,b)=>a+b, 0);
  console.log(N*(N+1)/2 - sum);
}

//-- functions ------------------------------------

function getIntegers(lines, sep) {
  if(sep === undefined) {
    return lines.map(function(e) {
      return Number(e);
    });  
  } else {
    return lines.shift().split(sep).map(function(e) {
      return Number(e);
    });  
  }
}

function getStrings(lines, sep=" ") {
  return lines.shift().split(sep);
}

function twoDimenArrayByValue(h, w, value) {
  const list = new Array(h);
  for(let i=0; i<h; i++) {
    list[i] = new Array(w).fill().map(function(e) {
      return value.constructor == Array ? Array.from(value) : value;
    });
  }
  return list;
}

function twoDimenArrayByStrings(h, strings) {
  const list = new Array(h);
  for(let i=0; i<h; i++) {
    list[i] = strings.shift().split("");
  }
  return list;
}

function distance(x1, y1, x2, y2) {
  return Math.sqrt((x2-x1) * (x2-x1) + (y2-y1) * (y2-y1));
}

function gcd(a, b) {
  return b === 0 ? a : gcd(b, a % b);
}

//-- Array -------------------------------------------

Array.prototype.sortByIntegerA = function() {
  this.sort(function(a, b) { return a - b; });
};

Array.prototype.sortByIntegerD = function() {
  this.sort(function(a, b) { return b - a; });
};

Array.prototype.orLess = function(V, l=0, r=this.length) {
  l--;
  while(r - l > 1) {
    let x = Math.floor((r + l) / 2);
    this[x] > V ? r = x : l = x;
  }
  return r;
};

Array.prototype.lessThen = function(V, l=0, r=this.length) {
  l--;
  while(r - l > 1) {
    let x = Math.floor((r + l) / 2);
    this[x] >= V ? r = x : l = x;
  }
  return r;
};

Array.prototype.orHigher = function(V, l, r) {
  return this.length - this.lessThen(V, l, r);
};

Array.prototype.greaterThan = function(V, l, r) {
  return this.length - this.orLess(V, l, r);
};

Array.prototype.deepCopy = function(list) {
  let retList = [];
  for(let i=0; i<list.length; i++) {
    retList[i] = list[i].constructor == Array ? Array.from(list[i]) : list[i];
  }
  return retList;
};

Array.prototype.out = function() {
  let outList = [];
  if(this[0].constructor == Array) {
    for(let i=0; i<this.length; i++) {
      outList.push(this[i].join(" "));
    } 
  } else {
    outList.push(this.join(" "));
  }
  console.log(outList.join("\n"));
};

Array.prototype.pushNoSameValue = function(...values) {
  values.forEach(function(e) {
    if(!this.includes(e)) {
      this.push(e);
    }
  }, this);
};

main(require("fs").readFileSync("/dev/stdin", "utf8").trim().split("\n"));