#include <iostream>
#include <cstdio>
using namespace std;

int main()
{
    int a, b, c[10000], n, p, sum = 0, i, k, s, m;
    scanf("%i %i", &a, &b);

    for(i = 1;i <= b ; i++)
    {
        scanf("%i", &c[i]);

        sum += c[i - 1];
    }
    k = sum - a * (b - 1);
    if(k <= 0) {
       m = c[b] - a;
       if(m <= 0)
        printf("0\n");
       else
        printf("%i\n", m);
    }
    else {
        n = (k + c [b]) - a;
        if(n <= 0)
            printf("0\n");
        else
            printf("%i\n", n);
    }
}

Edited by author 20.11.2014 20:03