#include <stdio.h>

int main(void) {
    int K,N,F,A[100],total,renu=0,answer;
    scanf("%d %d %d", &K, &N, &F);
    for(int i=0; i<F; i++) {
        scanf("%d", &A[i]);
        renu += A[i];
    }
    total = K * N;
    answer = total - renu;
    if(answer < 0) {
        answer = -1;
    }
    printf("%d", answer);
}