ioni 01

int prim (int n)
{
    int i;
    if(n<2||(n>2&&n%2==0))
        return 0;
    for (i=3; i*i<=n; i=i+2)
        if(n%i==0)
            return 0;
    return 1;
}

int i_prim(int n)
{
    int st=n,dr=n;
    while (prim(st)==0)
        st--;
    while (prim (dr)==0)
        dr++;
    return dr-st;
}