const input = require('fs')
  .readFileSync('/dev/stdin', 'utf8');
const [A, B, C, D] = input
  .split(/\s/)
  .map(v => +v);

let count = 0;
for (let i = A; i <= B; i++) {
  for (let j = C; j <= D; j++) {
    if (i !== j) count++;
  }
}
console.log(count);