let i = require('fs').readFileSync('/dev/stdin', 'utf8').split('\n')
let s = i[0].split(' ').map((e) => parseInt(e))
let f = i[1].split(' ').map((e) => parseInt(e)).reduce((p,c) => p+c)
let o = s[0] * s[1] - f
console.log(o < 0 ? -1 : o)