let lines = [];
let reader = require('readline').createInterface({
  input: process.stdin,
  output: process.stdout
});

reader.on('line', function(line) {
  lines.push(line);
});

reader.on('close', function() {
  let firstLine = lines[0].split(' ');
  let secondLine = lines[1].split(' ');

  const K = firstLine[0];
  const N = firstLine[1];
  const F = firstLine[2];
  let beans = K * N;

  for(let i = 0; i < F; i++) {
    beans -= secondLine[i];
  }

  if (beans >= 0) {
    console.log(beans);
  }
  else {
    console.log(-1);
  }
});