let main = (standardInput) => {
    var line = standardInput.trim().split("\n");
    var beansline  = line[0].trim().split(" ").map(Number);
    var membersline = line[1].trim().split(" ").map(Number);
    var beanssum = beansline[0] * beansline[1];
    var sum = membersline.reduce(function (accumulator, currentValue) {
    return accumulator + currentValue;
    }, 0);
    var ans = beanssum - sum;
    if (ans >= 0) {
        console.log(ans);    
    } else {
        console.log("-1");
    }
};
main(require('fs').readFileSync('/dev/stdin', 'UTF-8'));