import std.conv, std.functional, std.range, std.stdio, std.string;
import std.algorithm, std.array, std.bigint, std.bitmanip, std.complex, std.container, std.math, std.mathspecial, std.numeric, std.regex, std.typecons;
import core.bitop;

class EOFException : Throwable { this() { super("EOF"); } }
string[] tokens;
string readToken() { for (; tokens.empty; ) { if (stdin.eof) { throw new EOFException; } tokens = readln.split; } auto token = tokens.front; tokens.popFront; return token; }
int readInt() { return readToken.to!int; }
long readLong() { return readToken.to!long; }
real readReal() { return readToken.to!real; }

bool chmin(T)(ref T t, in T f) { if (t > f) { t = f; return true; } else { return false; } }
bool chmax(T)(ref T t, in T f) { if (t < f) { t = f; return true; } else { return false; } }

int binarySearch(alias pred, T)(in T[] as) { int lo = -1, hi = cast(int)(as.length); for (; lo + 1 < hi; ) { const mid = (lo + hi) >> 1; (unaryFun!pred(as[mid]) ? hi : lo) = mid; } return hi; }
int lowerBound(T)(in T[] as, T val) { return as.binarySearch!(a => (a >= val)); }
int upperBound(T)(in T[] as, T val) { return as.binarySearch!(a => (a > val)); }


/*
  (x2, y2): min y (< 0)
  (x1, y1): max y (x <= x2)
  oper [y1, y2)
  y1' >= 2 y1 - y2 > 2 y1
  ==> 1 + log_2(N) steps
*/

enum INF = 10^^9;
enum K = 20;

void main() {
  try {
    for (; ; ) {
      const N = readInt;
      auto S = readToken;
      
      auto dp = new int[2 * K + 1];
      dp[] = -INF;
      dp[0] = 0;
      foreach (i; 0 .. N + 1) {
        foreach (k; 0 .. 2 * K + 1) {
          if (dp[k] < 0) {
            dp[k] = -INF;
          }
        }
        foreach (k; 0 .. 2 * K) {
          chmax(dp[k + 1], dp[k]);
        }
        if (i < N) {
          final switch (S[i]) {
            case '+': {
              foreach (k; 0 .. 2 * K + 1) {
                dp[k] += (-1)^^(k & 1);
              }
            } break;
            case '-': {
              foreach (k; 0 .. 2 * K + 1) {
                dp[k] -= (-1)^^(k & 1);
              }
            } break;
            case '?': {
              dp[] += 1;
            } break;
          }
        }
      }
      
      int ans = K + 1;
      foreach (k; 0 .. K + 1) {
        if (dp[2 * k] >= 0) {
          ans = k;
          break;
        }
      }
      writeln(ans);
    }
  } catch (EOFException e) {
  }
}