const row = require('fs').readFileSync('/dev/stdin','utf8').split('\n');
const column = row.map(e => e.split(' ').map(e => Number(e)));

const sum = column[0][0] * column[0][1];
const eatBeans = column[1].reduce((acc, cur) => {
    return acc + cur;
});

let result = -1;
if(sum - eatBeans >= 0) {
    result = sum - eatBeans;
}

console.log(result);