function Main(input) {
    var data = input.split("\n");
    var line = data[0].split(" ")
    var K = +line[0];
    var N = +line[1];
    var F = +line[2];
    var A = data[1].split(" ").map(Number);

    rapt(K, N, F, A);
}

function rapt(K, N, F, A) {
    var ageSum = 0;
    var totalK = K * N;

    for (var i = 1; i <= F; i++) {
        ageSum += A[i-1]
    }

    if (totalK >= ageSum) {
        console.log(totalK - ageSum);
    } else {
        console.log(-1)
    }

}

Main(require("fs").readFileSync("/dev/stdin", "utf8"));