import java.io.*; import java.util.*; import java.util.Map.Entry; public class Main { final boolean isDebug = false; String fileName = "input.txt"; FastScanner sc; //PrintWriter out; final int MOD = (int)1e9+7; final int INF = Integer.MAX_VALUE / 2; /* solve */ void solve() throws Exception{ int A = sc.nextInt(); int B = sc.nextInt(); int C = sc.nextInt(); int D = sc.nextInt(); long ans = (B - A + 1) * (D - C + 1); if(B >= C && B <= D){ ans -= (B - C + 1); }else if(D >= A && D <= B){ ans -= (D - A + 1); } System.out.println(ans); } /* end solve */ /* main */ public static void main(String[] args) throws Exception { new Main().m(); } void m() throws Exception { long S = System.currentTimeMillis(); sc = (isDebug) ? new FastScanner(new FileInputStream(fileName)) : new FastScanner(System.in); //out = new PrintWriter(System.out); solve(); //out.flush(); long G = System.currentTimeMillis(); if(isDebug){ System.out.println("---Debug---"); System.out.printf("%8d ms", (G-S)); } } /* end main */ } /* end Main */ class FastScanner { private InputStream in; private final byte[] buffer = new byte[1024]; private int ptr = 0; private int buflen = 0; public FastScanner(InputStream in) { this.in = in; } private boolean hasNextByte() { if (ptr < buflen) { return true; }else{ ptr = 0; try { buflen = in.read(buffer); } catch (IOException e) { e.printStackTrace(); } if (buflen <= 0) { return false; } } return true; } private int readByte() { if (hasNextByte()) return buffer[ptr++]; else return -1; } private static boolean isPrintableChar(int c){ return 33 <= c && c <= 126; } public boolean hasNext() { while(hasNextByte() && !isPrintableChar(buffer[ptr])) ptr++; return hasNextByte(); } public String next() { if (!hasNext()) throw new NoSuchElementException(); StringBuilder sb = new StringBuilder(); int b = readByte(); while(isPrintableChar(b)) { sb.appendCodePoint(b); b = readByte(); } return sb.toString(); } public long nextLong() { if (!hasNext()) throw new NoSuchElementException(); long n = 0; boolean minus = false; int b = readByte(); if (b == '-') { minus = true; b = readByte(); } if (b < '0' || '9' < b) { throw new NumberFormatException(); } while(true){ if ('0' <= b && b <= '9') { n *= 10; n += b - '0'; }else if(b == -1 || !isPrintableChar(b)){ return minus ? -n : n; }else{ throw new NumberFormatException(); } b = readByte(); } } public int nextInt() { long nl = nextLong(); if (nl < Integer.MIN_VALUE || nl > Integer.MAX_VALUE) throw new NumberFormatException(); return (int) nl; } public double nextDouble() { return Double.parseDouble(next()); } }